Clarify GitHub Pages base config and withastro/action usage

[casungo]

Description

This PR updates the English documentation for deploying Astro sites to GitHub Pages (guides/deploy/github.mdx) to address several points of confusion and provide clearer, more accurate instructions.

These changes aim to significantly reduce confusion for users deploying to GitHub Pages subpaths and improve the reliability of their setups by promoting best practices for path handling.

Related issues & labels

• Closes (en) github.mdx: The description of the text is discrepancy. #10868
• Suggested label: improve or update documentation

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en/guides/deploy/github.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.