ENH: Support watching single files

[snowman2]

Context

In the base directory of a repository, there can be files such as CONTRIBUTING.md that are included in the docs by reference:

\```{include} ../CONTRIBUTING.md
\```

Proposal

It would be nice to be able to watch a specific set of files:

--watch *.md

Otherwise, there is going to be a long list of ignores.

Tasks and updates

No response

[choldgraf]

I've got a PR to add glob-style ignore matching here:

• Supporting glob-style operations #123

I think if that is merged, it should be pretty easy to implement this with logic like:

• Support --watch path/to/file.md or --watch path/to/*.md
• watch_files = glob(<that-expression>)
• all_files = glob("**/*", recursive=True)
• ignore_files = [ii for ii in all_files if ii not in watch_files]

And then append ignore_files to the list that is created if somebody used the --ignore flag.