Move Shell documentation into the main developer docs

Should we move the gitlab-shell documentation into the main developer guides?

I'm not really sure where it should go. Perhaps under Feature development, but that folder currently seems to be a simple list of various features.

It would be good to further develop the docs as:

• High level overview of Shell (both OpenSSH and gitlab-sshd versions)
  • Include lists of supported protocols etc
• Technical guide, including Kubernetes configuration
  • Contributor guides
  • How-to guides
• Links to all deep dives, blog posts on a single page etc

Step 1

As discussed on 2022-11-14 by @sean_carroll, @aqualls, @tlinz: add this item to 15.8, and Amy to work with @j.seto.

• @sean_carroll to assign his old MR to Amy -> Draft: Update README.md (!532 - closed)
• @aqualls to do the initial lift-and-shift from the external repo
• @aqualls to then incorporate @sean_carroll 's changes
• @aqualls to add a cross-link to @igor.drozdov 's blog post: https://about.gitlab.com/blog/2022/08/17/why-we-have-implemented-our-own-sshd-solution-on-gitlab-sass/

Step 2

• @j.seto can start revising in smaller, less painful iterations rather than trying to lift and shift the docs himself
• @j.seto / @aqualls to figure out what to steal from the blog post, rather than just cross-linking out to it