refactor: remove Segment plugin references and related configurations

[MarcosCA]

This commit removes all references to the Segment plugin from the documentation and configuration files, including the README, sidebar, and environment variables in GitHub workflows. The Segment plugin documentation file has also been deleted, streamlining the analytics integration options available in the project.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the removed Segment doc URL with a redirect

When the docs site deploys this deletion, existing links to /docs/plugins/plugin-segment (bookmarks, blog posts, search results) will start returning 404s because the page is removed outright here and docs/_redirects does not add a replacement route. This repo already maintains redirects for retired docs URLs, so adding one for the Segment page would avoid breaking inbound links while still hiding it from the sidebar.

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