Fix broken links

[kdp-cloud]

When using a baseurl in _config.yml, links to an absolute path (mostly images) do not resolve.

This will fix deployments of the documentation under a sub-URI or GitHub pages.

[kdp-cloud]

@PhilReedData I replace the site.baseurl with relative_url.

[PhilReedData]

Hi @kdp-cloud, I've added some suggestions where (a) I had not yet removed the .html bit from some links, and (b) some of the links had not yet been made to work with relative_url.

Still to do:

1. Add to the contribution guide a note about how to do links with relative_url and why.
2. Merge / sync latest changes from main.

Do you know how to do number 2? I can look at writing something for 1 but I don't know if I have permission to make changes directly to this branch.

[PhilReedData]

I've tested locally, images still work on all pages for me. I've written updates to the contribution guidance page to match these changes, but I don't have permission to push them. I'll apply them right after merging this PR.