docs: improve page actions configuration with clear disable example

[devin-ai-integration]

Summary

Improves the page actions configuration documentation to make it easier for users to disable page actions. The previous example was confusing because it:

• Set default: copy-page while also setting copy-page: false (contradictory)
• Didn't include view-as-markdown: false (which is enabled by default)
• Mixed disabling and enabling actions without clear explanation

This PR adds two clearly labeled sections:

1. Disable all page actions - A simple, copy-paste snippet that sets all three default-enabled options to false
2. Customize page actions - A coherent example showing how to disable defaults and enable a specific action

Review & Testing Checklist for Human

• Copy-paste the "Disable all page actions" YAML snippet into a test docs.yml and verify all page actions are hidden
• Copy-paste the "Customize page actions" YAML snippet and verify only the Cursor button appears
• Review the preview deployment to ensure the documentation renders correctly

Notes

Link to Devin run: https://app.devin.ai/sessions/75e660966f354fd0aa59880512a33fb0
Requested by: Fern Support (info@buildwithfern.com)

[devin-ai-integration]

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