LP1991444: Cleanup Build Docs Artifacts in GitHub Actions

LP1991444: Cleanup Build Docs Artifacts in GitHub Actions

The Build Docs GitHub action leaves the built docs behind as
artifacts.  These artifacts currently use about 58MB apiece.  This can
quickly consume all of the available storage spacce (500MB) for
free-tier GitHub accounts.

This commit adds a retention-days setting of 2 so that build docs
artifacts should be removed after two days.

See also:

https://docs.github.com/en/actions/using-workflows/storing-workflow-data-as-artifacts#configuring-a-custom-artifact-retention-period

Signed-off-by: Jason Stephenson <jason@sigio.com>