Add CMS Brew stub to Astro's CMS guides

[andrewchmr]

📚 Subject area/topic

CMS guides

📋 Page(s) affected (or suggested, for new content)

https://docs.astro.build/en/guides/cms/

📋 Description of content that is out-of-date or incorrect

This is a proposal for new content (a stub), not a correction.

I'd like to add a stub page for connecting Astro with CMS Brew (https://cmsbrew.com), a chat-based, Git-backed CMS for client-run sites. Like the existing Git-based CMS stubs (Sitepins, Pages CMS, GitCMS), you connect a GitHub repository and content edits are committed back to it, with no schema or config to write. It supports Astro, with a no-signup live demo at https://cmsbrew.com/demo.

It would sit alongside the other Git-based options in the CMS guide. I'm happy to open a PR with the stub modeled on the existing CMS stub format (e.g. sitepins.mdx), plus the logo asset and logos.ts entry, once this sounds good to the team.