Now recommend 3.11.7 to match the `#testing-a-builder-config-locally`
section.

This patch makes the following improvements:
* Corrects the suggestion that `bbenv` needs to be made within an
llvm-zorg checkout.
* Gives workarounds for following the instructions on a system with
Python 3.13 (it removed some long-deprecated libraries, which causes
problems).
* Adds a note about how some builder workflows involve checking out
llvm-zorg to retrieve additional scripts and gives guidance on how you
can still make and test local changes to those scripts when that's the
case.

Looks like a few different phrasings got mashed up together.

The bulletpoint list wasn't rendering properly due to a missing blank
line.

With <https://github.com/llvm/llvm-zorg/pull/289> and <https://github.com/llvm/llvm-zorg/pull/293> landed, it's now reasonable to ask people to test their builder configurations locally. This patch adds documentation on how to do so.

Choosing another term for this one document would only create confusion,
and vendoring Buildbot to change it is a lot of work (as explained in
the linked Buildbot issue).

I spent a long time trying different combinations of primary and
verified emails, until a colleague tried it who happened to have a
public profile email set when I did not.

Document how this works to save everyone else the legwork.