CI: Check for broken links

[rht]

To prevent the problem solved by #1012 from happening, a script to check for broken links can be used. There is already such script in Zulip repo that can be used: https://github.com/zulip/zulip/blob/master/tools/test-documentation.

Description of the tool can be found at https://github.com/zulip/zulip/blob/master/docs/testing/testing.md#other-test-suites

- `tools/test-documentation`: Checks for broken links in this
  ReadTheDocs documentation site.