Adding Git Xet docs

[jsulz]

This PR brings docs about Git Xet into the core Hub docs. A few notes:

• I linked directly to Windows installers. Navigating GitHub to get to these downloads is clunky IMO. However, this creates link-update overhead on each release. Very open to alternatives if anyone has them.
• We're using a homebrew tap for Homebrew installs. We still need a PR to homebrew-core to get a more simple brew install. @seanses plans to handle this in an upcoming release.
• Install-only guidance for now as deeper “how it works” may change soon.
• Only one pointer in repositories-getting-started to git-xet while we roll out. My feeling was to add more once it stabilizes, but also open to feedback here if we think it's better to add callouts more liberally.

Also did some Markdown header cleanup from the restructure in #1969 sorry for the extra changes cluttering up this PR as a result.

Happy for any other feedback! 🤗

[HuggingFaceDocBuilderDev]

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[rajatarya]

One minor comment and one suggestion for handling windows download links.