Call for review: Move secondary requirement texts out of the background

Hey folks,
This pull requests refines and updates how secondary requirements are
presented in ACT Rules, moving their explanation from the understanding
document into the accessibility requirements section itself. This PR also
further standardizes what language to use for secondary requirements.

If you have any comments, please leave a review before August 30th:
https://github.com/act-rules/act-rules.github.io/pull/2060

-- 
*Wilco Fiers*
Axe-core & Axe-linter product owner - WCAG 3 Project Manager - Facilitator
ACT Task Force