Wording updates for requirements

[alastc]

This PR implements the updates from w3c/wcag3#44 which is mostly re-wording of the main requirements section.

The intent is to the make each requirement clearer and more testable, as WCAG 3 will be assessed against these requirements before final publication.

This PR also contains a couple of additions to the "Design principles" section from issue #371 and #307 that had already been agreed elsewhere. (Lines 188 and 189.)

Closes w3c/wcag3#44

Closes #727

[jspellman]

Repeat of the comment in the review:

Flexible maintenance and extensibility
Create a maintenance and extensibility model for guidelines that can better meet the needs of people with disabilities using emerging technologies and interactions.

This original requirement came from the Problem Statements that arose from the Silver Research project. See the section on Flexible Maintenance from the Problem Statements.

This has been watered-down to only apply to informative content. That is not the intention of this requirement. I propose the wording be changed to "The guidelines and documentation each include a process for updating and maintaining the guidance."

Maintenance has to include more than just the informative documentation. We need a way to update the guidelines that allows the guidelines to adapt to rapidly changing technology.

[alastc]

Hi @jspellman, we updated the wording from the discussion in meeting, could you approve this PR now please?
(I can't approve my own PR, or merge until reviewed...)