docs: reorganize docker-compose docs, clean up admin homepage

Created by: bobheadxi

For #22427

An idea for organizing installation method documentation, inspired by RFC 414:

• Sourcegraph with X
  • index
    • Installation
    • About: links to background info, some basic ideas, etc
  • operations
    • action-oriented FAQ of sorts - "Do X" instead of "How to do X?" to encourage agnostic snippets
  • ...cloud-specific guides
    • refer to operations as much as possible
  • migration to X

Try it out:

sg run docsite -> http://localhost:5080/admin/install/docker-compose

Ideas

General ideas from RFC 414 applied here (also see Distributions thoughts on docs):

• make install/docker-compose the source of truth regarding doing things with docker-compose
• focus on composable "operations" that we can easily reference throughout our docs (e.g.) instead of repeating in many pages
• reduce information spread (less pages, more stuff on big pages) because these are easier to maintain and discover
  • docs spread and repeated over many pages are hard to keep up to date. every page where information exists or is repeated is a page every single person has to know exists to keep up to date and know about to avoid adding even more information spread
• action-oriented rather than use-case-specific (more "Do X" instead of "How to do X?" to encourage agnostic snippets) that can then be referenced from "story docs" like https://github.com/sourcegraph/sourcegraph/pull/22907)
  • e.g. https://github.com/sourcegraph/sourcegraph/pull/23029#issuecomment-884603490, https://github.com/sourcegraph/sourcegraph/pull/22778/files#diff-8af2490cccfa363372820d89a6a0d3c6003067ebcb2ddb3678d2e873dee08939R9