Add command to check for vX.Y.Z tag vs pybind11/_version.py consistency.

[rwgk]

Description

Slightly expand instructions based on lesson just learned (see #4756).

Piggy-backing hints for converting changelog to release message.

Suggested changelog entry:

[rwgk]

Hi @henryiii did you get a chance to look at this tiny update?

I just double-checked the git diff command, with a fresh eye, practicing a mock release. It does work as intended:

• If the branch tag corresponding to what's in pybind11/_version.py does not exist:

$ git diff "$(grep ^__version__ pybind11/_version.py | cut -d'"' -f2 | sed 's/^/v/')"
fatal: ambiguous argument 'v2.11.9': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

• if the branch tag exists but the tagged commit is not identical to the currently checked-out git working copy it shows the diff.

I think that's exactly what we need as a last line of defense, right before the git push --tags in the instructions.

[rwgk]

@henryiii I removed a few line breaks to make it look better in the web view.

https://github.com/pybind/pybind11/blob/2b54c6539951cbd819b00b2140182bcf9a100ed7/docs/release.rst

Do you know if we can have the line breaks in the .rst file without making it look weird in the web view?

It's still not perfect, but good enough?

[rwgk]

It's still not perfect, but good enough?

With a lot of trial and error I finally figured out then main problem:

This is NOT GOOD:

- Bullet nesting level 1.
    - Bullet nesting level 2.

This is BETTER:

- Bullet nesting level 1.
  - Bullet nesting level 2.

With that and some minor miscellaneous polishing the rendered page on the web looks nice now. I'll go ahead and merge.