Add make linkcheck step, to check external links [#29]

[genehack]

Description of proposed changes

Adds a make linkcheck step to the Sphinx CI.

Note that this blocks on the various PRs attached to nextstrain/docs.nextstrain.org#14 — those PRs should be merged before this one.

Update to add: I realized this workflow is used by more than docs.nextstrain.org, so before this can be merged, the following repos need to be audited for broken links:

• Audit augur -- Fix broken links [#1569] augur#1570
• Audit auspice -- Fix broken links [#1807] auspice#1808
• Audit cli -- Fix broken links [#388] cli#389
• Audit ncov -- Fix broken links [#1133] ncov#1137

Related issue(s)

Closes #29
nextstrain/docs.nextstrain.org#14

Checklist

• Checks pass

[joverlee521]

I think we'd want to add the new make linkcheck step the the build-conda job as well?

[genehack]

I think we'd want to add the new make linkcheck step the the build-conda job as well?

Good call, done.

Also removed env-var and adjusted use of make-target.

[genehack]

Updated description above to note that before this can land, everything that uses it (listed above) needs to be manually audited to get make linkcheck to run cleanly. Links to open issues above.

[genehack]

This is now ready to merge — all the affected downstream repos should have clean make linkcheck runs in their doc trees — but I'm going to wait until Monday to actually push the button, just in case something goes pear-shaped after all.

[genehack]

This is now ready to merge — all the affected downstream repos should have clean make linkcheck runs in their doc trees — but I'm going to wait until Monday to actually push the button, just in case something goes pear-shaped after all.

I also validated a branch in each of the five repos that consume this PI action, changing docs-ci.yaml to point to this branch, and verifying that the CI remained green.