Docs: clean up integration documentation

[agjohnson]

I'll fill this out more, but I started on some clean up here and it turned into a lot of moving around content and deleting content.

Briefly, some of the changes are:

• Frame our integration documentaiton as project integration, not global "this is how you connect your account to Read the Docs". There is user guides for connecting accounts that is separate
• Add actual content for other integrations, especially GHA
• Break out troubleshooting content page that is basically a more informative version of our repo repair UI

[agjohnson]

Here's how the docs are currently organized

docs:
  /reference/git-integration:
    - Introduction of social connections
    - Getting started connected a social account
    - How automatic configuration works
    - Read the Docs incoming webhook
    - How does the connection work?
    - Git provider integrations
    - Permissions for connected accounts
      - GitHub, GitHub App, Bitbucket, GitLab
    - GitHub permission troubleshooting
    - GitHub App
      - GHA Features
      - GHA Adding a project from a repo
      - GHA Connect a repository
      - GHA Manually migrating
      - GHA Revoking access
      - GHA Security
      - GHA Troubleshooting
  /intro/add-project:
    - How to use the project create page forms, very specifically.
    - Auto add project form
    - Manually add project form
    - Doesn't mention private repos at all
  /guides/setup/git-repo-manual:
    - Manual integration setup
    - Payload validation
    - Managing integrations
    - Using the generic API integration
      - Parameters
      - Auth
    - Troubleshooting Git provider webhooks
  /guides/creating-project-private-repository:
    - In _Security and access_ in TOC
    - Low in results for "private" search query, needs bump in score

Here is what I'm thinking of landing at after my first stabs at moving some content around -- in YAML for some reason I'm still not sure of:

docs:
  # There is currently some focus on how to connect an account on the
  # integration page and that makes the Git integration concept harder to
  # explain and it bloats the page more. Instead, make this separate if it isn't
  # already.
  /reference/connected-services:
    - Introduction of social connections
    - Getting started connected a social account

  /reference/git-integration:
    # These sections sort of exist, but the focus of the content is a bit confusing.
    # Some sections need to be refocused more generically to describe both GHA
    # and Oauth integration.
    - What do integrations do?
    - Integration types
    - Integration providers

  # These are likely okay on the parent /ref/git-integration page, but it
  # might feel natural breaking these out, especially with a page like the manual
  # provider reference page.
  /reference/git-integration/oauth:
    - Oauth features
    - How integration works (webhook)
  /reference/git-integration/github-app:
    - GHA Features
    - How integration works (gha not webhook)
    - Security
  /reference/git-integration/manual:
    - How integration works
    # From /setup/git-repo-manul section using the generic API integration
    - Using the generic API integration
      - Parameters
      - Auth
  # This is most likely to be broken out though, there are some sections in the
  # parent file that will differ based on provider.
  /reference/git-integration/providers:
    - Git provider integrations supported integration types
      - GitHub
      - GitLab
      - Bitbucket
      - Manual support for unsupported
        - Maybe actual instructions for Gitea, Forgejo, Azure, etc?
    - Permissions for connected accounts

  # This page could use help, it's pretty empty for an introduction and we have
  # better content like /guides/creating-project-private-repository that
  # actually show the steps. I might suggest merging the private repo guide into
  # this page and mentioning private repo steps as optional for business users
  # only. Or, move content from the private repo page and reduce that content.
  /intro/add-project:
    - How to use the project create page forms, very specifically.
    - Auto add project form
    - Manually add project form
  /guides/creating-project-private-repository:
    - In _Security and access_ in TOC
    - Low in results for "private" search query, needs bump in score

  # Rename for consistency, from /guides/setup/git-repo-manual.
  # This page is not for describing manual generic API integration type anymore.
  /guides/git-integration/manual-setup:
    - Manual integration setup
    - Managing integrations
    - Payload validation

  # Break some specific topics out to guide pages to reduce scope of reference pages
  /guides/git-integration/migrating-github-app-manually:
    - GHA Manually migrating
  /guides/git-integration/managing-github-app:
    - GHA Adding a project from a repo
    - GHA Connect a repository
    - GHA Revoking access

  # Unsure if this makes more sense as a separate page, it might just be a
  # section on individual reference pages. However there is some content that is
  # applicable to _all_ git integrations, so it feels better close to
  # troubleshooting steps for other providers.
  /guides/git-integration/troubleshooting:
    - Troubleshooting integrations 
    - Oauth troubleshooting
    - GHA Troubleshooting