Move PX4 Guide source into /docs

[hamishwillee]

This is first steps to moving docs into PX4-Autopilot tree:

• Vitepress
  • Copy in core folders
• Github
  • .gitignore - done in subtree
  • .gitattributes - may be problem - sources currently have CRLF.
• Source for docs
• Workflows
  • Stop PX4 build/test workflows running for docs/ path
  • Crowdin upload/download
  • Flaw checker
  • Docs deploy
  • Add the required secrets to allow deployment, if needed
• Add doc source

@mrpollo Trying to get some last minute PRs before I push this in. Two things worth looking at now, and we could push this in early:

• Confirm you are OK with having this in /docs path. After all, there is a Documentation folder.
• Worth looking at the workflow changes I have made to existing PX4 workflows just to confirm they make sense to you. Once those are in will reduce the churn made as I extend/test this.

[hamishwillee]

We still need this while we have legacy gitbook in play - it provides the default version switcher options. Would love to have this same approach for vitepress. There's a plugin that does similar, but I don't trust it.

[mrpollo]

Great work, @hamishwillee. I like the setup; I made minor changes to the ci workflow files and can't wait to have the docs here!

[hamishwillee]

Merging sources - let's get this building (now sources are present, even viewing this PR is impossible). I'll test the integration after.