Add link testing, label syncing, and issue labeling

[mgwalker]

This pull requests adds some automations, like what has been previously added to the UX guide:

• verify that there are no root-relative links in the source code
• verify that there are no invalid internal links in the build HTML
• synchronize labels with the UX guide repo nightly
• add the "new issue" label to newly-created issues, so maintenance teams can quickly know what has/has not been triaged

[github-actions[bot]]

At least one link is invalid. Please check the output of the tests in the checks below. For more information, see the wiki page on automated tests.

[github-actions[bot]]

At least one link is invalid. Please check the output of the tests in the checks below. For more information, see the wiki page on automated tests.

[github-actions[bot]]

At least one link is invalid. Please check the output of the tests in the checks below. For more information, see the wiki page on automated tests.

[mgwalker]

Requesting review:

• @cannandev: Engineering review, specifically of the GitHub Actions code

Associated issues:

• opened #578 to fix the root-relative links
• opened #579 to see which HTTP links can be changed to HTTPS

[github-actions[bot]]

At least one link is invalid. Please check the output of the tests in the checks below. For more information, see the wiki page on automated tests.

[github-actions[bot]]

At least one link is invalid. Please check the output of the tests in the checks below. For more information, see the wiki page on automated tests.