Support auto-linking #... issue references to pantsbuild/pants

[huonw]

In pantsbuild/pants#20492 (comment), we noticed that it's convenient to write #... issue references in docs, and some places (particularly GitHub renderers) will auto-link them.

If we could do the same in our published website that might be nice, e.g. replace matches of /#([0-9]{3,})/ with [#\1](https://github.com/pantsbuild/pants/issues/\1) (or some equivalent). Notes:

• Preferably we'd do this in a Markdown aware-way, only in body text, and not within code blocks, existing links etc.
• The {3,} is to require at least #123, to reduce spurious references like a reference to a list item ("#1 is a good option").

From a quick skim of the docusaurus docs and plugins, there doesn't appear to be existing support for this, but I might've missed it.

[thejcannon]

This should be doable in a remark plugin

[huonw]

Ah, nice! Looks like https://github.com/remarkjs/remark-github exists already and https://docusaurus.io/docs/markdown-features/plugins seems to describe how to install it.