[guide] ARC Best practices

[Brilator]

Add a Guides section on "best practices" or "how to work with the ARC"

Most guides and instructions so far are pretty much first steps and "forward" only.
We haven't really addressed what's happening "in the middle" once a user has created an ARC and started using it.

• syncing practices
  • recommendation for how often to arc sync
• use of commit messages
  • protocols the development of the ARC, i.e. aligns with the project
  • screenshot of branches
• practical guide to add "legacy data" #181
• adding external / unpublished data
  • already covered in FAQ, but might make sense to emphasize this here
• use of gitignore
  • e.g. to keep sensible, unpublished data from sharing
  • note: files added to gitignore on one machine will neither be synced to the DataHUB nor tracked by version control
  • note: if a file was already in the DataHUB, using gitignore will only keep it from future syncs and not remove it
• avoiding download of LFS objects to small computers