docs-ci: Check external links

[tsibley]

Running Sphinx's linkcheck builder to systematically check our external links would be really helpful to catch broken links resulting from anything from typos when authoring to third-party site reorganizations.

It would have caught a bunch of external link errors in nextstrain/docs.nextstrain.org#139, for example. (Not calling out that PR specifically! This sort of error is very common and easy to make. But that PR made me wonder why we weren't already running linkcheck.)

A second job (for parallelism) or just a subsequent step in the current job should run something like:

SPHINXOPTS="-W --keep-going" make linkcheck

which will produce a bunch of messages to stdout/stderr output but also build/linkcheck/output.{txt,json}. Both of the latter files (and probably just the whole build/ dir!) should be uploaded as a workflow artifact. Examples of output from nextstrain/docs.nextstrain.org@6fcf76f.

Since linkcheck will exit with a non-zero status on errors, it'll fail the CI job. I believe this is what we want.

Some configuration for linkcheck is usually required to ignore links that are actually ok, and this would fall on each doc project to configure itself.

[genehack]

@tsibley is this still something that needs to be done?

[tsibley]

I think it's (still) worth doing. Clearly not high priority, but fairly low effort to actually do.