Add command to check for vX.Y.Z tag vs pybind11/_version.py consistency. (#4757)

* Add command to check for vX.Y.Z tag vs pybind11/_version.py consistency. Piggy-backing hints for converting changelog to release message.

* Simpler way of double-checking version number in sources vs. git tag

* Even simpler.

* Fix rst rendering (hopefully) and remove stray dots.

* [ci skip] Trying more to make GitHub rst renderer show this nicely.

* [ci skip] Fix up RST rendering issues.

Lesson learned:

This is NOT GOOD:

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

This is BETTER:

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

Also consistently adding empty lines between bullet points, to make the .rst
file easier to read.

Also piggy-backing a few very minor enhancements.