Simplify/correct definition links

[patrickhlauke]

Closes #3920

[kfranqueiro]

This looks good to me for techniques and understanding. I would suggest removing the changes in this PR for conformance-challenges and guidelines since those aren't processed the same way. Sorry I was probably ambiguous about that in the issue.

[patrickhlauke]

Reverted those two non-technique/non-understanding changes

[kfranqueiro]

TL;DR: we should not merge this until we're using the Eleventy build process, where I've fixed an underlying issue.

This isn't so much a problem with this PR as it is with the XSLT build process (the same issues already exist elsewhere in content). In the process of thoroughly testing, I think I understand why some of these (particularly some under techniques) were using full URLs.

The XSLT build process generates its own IDs for definitions links, disregarding the already-existing id attributes in the markup in guidelines/terms/* files. This is fine for Understanding pages, which cross-link to headings under its own Key Terms section. However, Techniques pages don't do this; they link directly to the TR - which is bad if the IDs don't match.

As it stands, the links to "status messages", "change of context", "conforming alternate version", and "web pages" will not scroll to the correct position in the TR when this PR's changes are processed by the XSLT build process. I have added a commit to the Eleventy build process PR (#3917) to address this for Techniques pages, so this PR will work 100% with that build process. (The fix also resolves 16 existing instances of this problem, so it's great that this PR helped catch them!)

[kfranqueiro]

Tthe Eleventy build process has now been successfully used to push the latest site update, and merged into the main branch, so it's now safe to merge this PR.

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