Fix version-specific GH link in customizing_html5_shell.rst

[xhocquet]

The current link points to main, which cost me a couple hours as I tried to figure out errors that didn't belong to Godot 3! There might be value in find + replacing similar links in the docs and pinning them to tagged versions of github files, instead of their main versions

[xhocquet]

Let me know if there's any interest in this type of change, and I'd be happy to put up PRs for further updates! If not, no worries 🙏🏻

[Calinou]

List of links that could be replaced (rg "/blob/master"):

development/file_formats/tscn.rst
19:`resource_format_text.cpp <https://github.com/godotengine/godot/blob/master/scene/resources/resource_format_text.cpp>`_

development/cpp/custom_audiostreams.rst
25:-  `servers/audio/audio_stream.h <https://github.com/godotengine/godot/blob/master/servers/audio/audio_stream.h>`__
26:-  `scene/audio/audio_stream_player.cpp <https://github.com/godotengine/godot/blob/master/scene/audio/audio_stream_player.cpp>`__
119:-  `servers/audio/audio_stream.h <https://github.com/godotengine/godot/blob/master/servers/audio/audio_stream.h>`__
349:-  `core/math/audio_frame.h <https://github.com/godotengine/godot/blob/master/core/math/audio_frame.h>`__
350:-  `servers/audio/audio_stream.h <https://github.com/godotengine/godot/blob/master/servers/audio/audio_stream.h>`__
351:-  `scene/audio/audio_stream_player.cpp <https://github.com/godotengine/godot/blob/master/scene/audio/audio_stream_player.cpp>`__

development/cpp/custom_resource_format_loaders.rst
21:- `core/io/resource_loader.cpp <https://github.com/godotengine/godot/blob/master/core/io/resource_loader.cpp>`_
41:- `core/io/image_loader.h <https://github.com/godotengine/godot/blob/master/core/io/image_loader.h>`_
307:- `core/io/fileaccess.h <https://github.com/godotengine/godot/blob/master/core/os/file_access.h>`_
358:- `core/io/resource_loader.cpp <https://github.com/godotengine/godot/blob/master/core/io/resource_loader.cpp>`_

development/cpp/custom_godot_servers.rst
376:- `servers/register_server_types.cpp <https://github.com/godotengine/godot/blob/master/servers/register_server_types.cpp>`__

development/compiling/introduction_to_the_buildsystem.rst
355:`version.py file in the Godot Git repository <https://github.com/godotengine/godot/blob/master/version.py>`__.

about/complying_with_licenses.rst
100:`third parties <https://github.com/godotengine/godot/blob/master/COPYRIGHT.txt>`_.

about/faq.rst
27:For full details, look at the `COPYRIGHT.txt <https://github.com/godotengine/godot/blob/master/COPYRIGHT.txt>`_ as well
28:as the `LICENSE.txt <https://github.com/godotengine/godot/blob/master/LICENSE.txt>`_ and `LOGO_LICENSE.txt <https://github.com/godotengine/godot/blob/master/LOGO_LICENSE.md>`_ files
388:Find an issue that resonates with you, then proceed to the `How to Contribute <https://github.com/godotengine/godot/blob/master/CONTRIBUTING.md#contributing-pull-requests>`_
399:core functionality as outlined in the `Roadmap <https://github.com/godotengine/godot-roadmap/blob/master/ROADMAP.md>`_,

_templates/layout.html
2:{# Refer to https://github.com/readthedocs/sphinx_rtd_theme/blob/master/sphinx_rtd_theme/layout.html #}

classes/class_editorsettings.rst
1939:- **Light** makes fonts and icons dark (suitable for light themes). Icon colors are automatically converted by the editor following `this set of rules <https://github.com/godotengine/godot/blob/master/editor/editor_themes.cpp#L135>`__.
2095:The SSL certificate bundle to use for HTTP requests made within the editor (e.g. from the AssetLib tab). If left empty, the `included Mozilla certificate bundle <https://github.com/godotengine/godot/blob/master/thirdparty/certs/ca-certificates.crt>`__ will be used.

tutorials/inputs/controllers_gamepads_joysticks.rst
16:<https://github.com/godotengine/godot/blob/master/CONTRIBUTING.md#reporting-bugs>`__.

tutorials/platform/customizing_html5_shell.rst
22:`/misc/dist/html/full-size.html <https://github.com/godotengine/godot/blob/master/misc/dist/html/full-size.html>`__

tutorials/platform/ios/plugins_for_ios.rst
93:Implemented in `Godot iOS InAppStore plugin <https://github.com/godotengine/godot-ios-plugins/blob/master/plugins/inappstore/in_app_store.mm>`_.
245:Implemented in `Godot iOS GameCenter plugin <https://github.com/godotengine/godot-ios-plugins/blob/master/plugins/gamecenter/game_center.mm>`_.

tutorials/export/changing_application_icon_for_windows.rst
111:    `platform/windows/godot.ico <https://github.com/godotengine/godot/blob/master/platform/windows/godot.ico>`__

tutorials/shaders/shaders_style_guide.rst
14:`here <https://github.com/godotengine/godot/blob/master/drivers/gles2/shaders/copy.glsl>`__.
33:    // https://github.com/godotengine/godot-demo-projects/blob/master/2d/screen_space_shaders/shaders/BCS.shader

classes/class_projectsettings.rst
5613:The CA certificates bundle to use for SSL connections. If this is set to a non-empty value, this will *override* Godot's default `Mozilla certificate bundle <https://github.com/godotengine/godot/blob/master/thirdparty/certs/ca-certificates.crt>`__. If left empty, the default certificate bundle will be used.

tutorials/scripting/gdscript/gdscript_basics.rst
137:Keywords are defined in the `GDScript tokenizer <https://github.com/godotengine/godot/blob/master/modules/gdscript/gdscript_tokenizer.cpp>`_

community/tutorials.rst
8:Think there is something missing here? Feel free to submit a `Pull Request <https://github.com/godotengine/godot-docs/blob/master/community/tutorials.rst>`_ as always.

community/contributing/code_style_guidelines.rst
29:`.clang-format <https://github.com/godotengine/godot/blob/master/.clang-format>`__

Not all of these should necessarily be replaced, but it's worth keeping in mind.

[xhocquet]

@Calinou do you agree that these changes are worthwhile? If so I can go through and use my best judgement on the ones you have listed