python / devguide

Expand the section on renaming to the 'main' branch by including instructions to rename local forks on GitHub.

This repeats points currently said in `Include/README.rst`.
I intend to replace most of that README with a link to this document.

The main change from that document are:
- The three "rings" are not defined by where things are declared.
  It would be good to have them in the right place, but it's
  not yet the case, and for technical reasons we might never reach
  the ideal. (In `Include/README.rst` it makes more sense to make
  the locations more prominent.)
- The order is switched. For changing the Limited API you generally
  need to read (and follow guidelines in) the preceding sections
  as well.

On top of that, I addes points from PEP 652 (Maintaining the Stable ABI).


Co-Authored-By: Erlend Egeberg Aasland <erlend.aasland@innova.no>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

set continue-on-error true on the linkcheck step

The repo changed name, from "performance" to "pyperformance", a while back.  The devguide did not get updated at that time.  Updating the links here means GitHub will no longer need to rewrite the old URL in requests.

cc @ewdurbin.

Co-authored-by: Andrew Kuchling <akuchling@aledade.com>

Closes #520