[DOCS] Add TextRerankPipeline to readme.md

[as-suvorov]

Description

Add TextRerankPipeline to readme.md
Align embedding and rerank models export flags

Checklist:

• I have made corresponding changes to the documentation.

[Copilot]

Pull Request Overview

This PR adds documentation for the TextRerankPipeline feature to the main README and standardizes the model export flags across documentation. The changes replace --trust-remote-code with explicit --task parameters for better consistency.

Key changes:

• Added TextRerankPipeline documentation with Python and C++ usage examples
• Standardized Optimum CLI export commands to use --task flag instead of --trust-remote-code
• Updated embedding model exports to use --task feature-extraction and rerank model exports to use --task text-classification

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Added complete TextRerankPipeline section with API examples and updated embedding model export command
site/docs/use-cases/text-rerank/index.mdx	Updated rerank model export to use --task text-classification
site/docs/use-cases/text-embedding/index.mdx	Updated embedding model export to use --task feature-extraction
samples/python/rag/README.md	Standardized export commands for both embedding and rerank models, removed trust_remote_code parameter
samples/js/rag/README.md	Updated embedding model export command to use task flag
samples/cpp/rag/README.md	Standardized export commands for both embedding and rerank models

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