(Split from #1570.)

This PR adds two reStructuredText lint rules and runs them on the CI using GitHub Actions. Example run.

1. Detect single backticks, which should be double in RST for inline code, e.g. stuff fixed in #1554.

2. Detect inline code touching normal text, e.g. stuff fixed in #1560.

This uses the pre-commit tool for linting. It's possible, but completely optional, to use it locally and it'll check your changes when you're committing, and fail before commit if something is found.

Optional local set up (run once):

pip install -U pre-commit && pre-commit install

Then make changes and commit as normal:

git commit -m "PEP XXX: etc"

Lint all files:

pre-commit run --all-files

# Or this does the same thing
make lint

The very first run takes a little while to set things up, after that it's quick.

But again, local use of pre-commit is completely optional for those who aren't keen, and the CI will run the checks anyway.

The first lint rule finds some false positives:

pep-0012.rst:602:    `single-quoted' or ``double-quoted''
pep-0505.rst:245:    Total None-coalescing `if` blocks: 449
pep-0505.rst:246:    Total [possible] None-coalescing `or`: 120
pep-0505.rst:248:    Total Safe navigation `and`: 13
pep-0505.rst:249:    Total Safe navigation `if` blocks: 61
pep-0550.rst:540:    g = gen()  # gen_LC is created for the generator object `g`
pep-0550.rst:763:        # This class is what the `gen_series()` generator can
pep-0550.rst:772:            # Otherwise `var.set(10)` in the `_init` method
pep-0550.rst:857:        # in the `Context Variables` section.

• pep-0012.rst is an example of what not to do! So that needs to stay.

• pep-0505.rst is example output from a command, so (most likely) needs to stay.

There's no way to exclude single lines from the linter, so both these files have been excluded.

• Backticks already in code: I've changed them to single quotes. Because it's already formatted with code, no extra formatting was added in the rendered monospace output, so single quotes can also be used for the emphasis.

• After rebasing on master, this found a few other single backticks which should have been double backticks or links. I've fixed them.