Adding warnings and

• links to the transition guide https://github.com/sagemath/trac-to-github/blob/master/docs/Migration-Trac-to-Github.md

As of this ticket (for Sage 9.8, assuming it can be released in January 2023), we keep the Trac instructions.

In a follow-up ticket (for Sage 9.9), we will remove the trac instructions, update the developer manual according to the transition guide, and add links to github command-line interface, e.g., https://cli.github.com/manual/gh_repo_fork

Part of Meta-ticket #30363: Migration from Trac to GitHub

CC: @kwankyu @soehms @tobiasdiez

Component: documentation

Author: Matthias Koeppe, Kwankyu Lee

Branch/Commit: cecbcac

Reviewer: Kwankyu Lee, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34648