fix broken links in r21 webdocs

Checkout r21. Fork a feature branch. Run hugo server --disableFastRender. This creates a website under an address like http://localhost:1313/21/.

Install linkchecker (Debian package has same name). Run linkchecker http://localhost:1313/21/. Ignore the warnings. Resolve the errors. Submit MR against r21.

After the MR is merged, login as www on scg3. cd ba/21/repo/hugo. Make sure we are on branch r21. Do git pull --all. Run hugo. Check that the changes are online at bornagainproject.org/21.