Look for external sources for Gitea documentation

[jemrobinson]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates
• [x] This is a request for a change to documentation
• [x] This isn't a feature request or bug report
• [x] This isn't an open-ended question (open a discussion if it is).

:ledger: Where is the problem?

Currently we maintain documentation and images about how to perform some common Gitea tasks. It would be nice if we could link to external sources, but Gitea's end-user documentation is poor.

:memo: Required documentation changes

Find replacements for:

• Create a new repository
• Clone an existing repository
• Create a pull request
• Create a project board (currently https://blog.gitea.com/introducing-new-features-of-labels-and-projects/)