Updated techniques

[mattgarrish]

This pull request updates the techniques to reflect the variety of changes over the last year, as detailed in issues #2621, #2626, and #2627.

I also made a minor editorial change to make the section ids more easily referenceable. I've kept the old ids for now on spans, but ideally these should disappear in time. (They used to match text identifiers we had for each technique but then we dropped those identifiers in 1.1.)

I'm going to give the document another pass to check that the other guidance we have still makes sense.

Fixes #2621 Fixes #2626 Fixes #2627

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[mattgarrish]

The last commit changes this sentence in the hazards section:

Individual unknown hazards must be definitively determined, report specified alongside "no hazard" values.

This isn't what I meant to say, as you could have hazard or no hazard values along with the individual unknowns. What I wanted to convey is that when setting individual hazards, all three need to be covered. I've replaced that sentence with this at the end of the section:

EPUB creators must ensure that information about all three types of hazards is included when not using the "unknown" or "none" values.

I also made a minor tweak to list each no hazard and unknown hazard value so you don't have to go searching in the vocabulary.

[mattgarrish]

As we're heading into summer break time for a lot of people, it's probably worth merging this now as there hasn't been any additional feedback. We can always announce the changes to the lists if we want to broadcast the updates a bit further.