Improve amendment management

[dontcallmedom]

This offers a significantly improved infrastructure to manage the amendments to the WebRTC Recommendation.

In particular:

• the diff that are displayed for amended sections are now granular, rather than the block-by-block diff used in the previous system
• errors in marking up amendments now make use of the ReSpec error mechanism, which should make it easier to identify them, including in the CI system
• the markup needed to indicate a complete new section is much simpler (no longer needed to add in base-rec.html, and only need to reference the specific section being added) - this follows from discussion with @Orphis in a recent instance

In addition, this fixes a number of amendments that had not been properly classified.

speced/respec#4786 will be needed to ensure the resulting generated markup is valid.

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[dontcallmedom]

this is now ready to be merged and would benefit from being merged rapidly since it impacts other pull requests documenting amendments (e.g. #2988) cc @jan-ivar @alvestrand