Improve wrapper handling for how-to links

[kfranqueiro]

This refactors the handling for how-to links to resolve multiple issues:

• It expected to find at least one leading paragraph or list element, which would throw an error if e.g. a Note comes first
• It would wrap multiple consecutive elements within the body-wrapper element, which would produce the wrong layout
• The processing occurs at the time the guideline/requirement's content is resolved, meaning the link would also end up inserted into informative docs in the future without additional work to pry it out
• The identified elements would be split into a separate frontmatter property, requiring cobbling the two together to output the full text, which I thought might be useful down the road but maybe isn't

This PR moves the processing from within a remark plugin, to specifically within the EntryText component, using Cheerio which is probably more likely to be readable to more people (since it effectively follows jQuery's API). This does not add any new dependency; Cheerio was already used for an environment variable that makes upgrades easier to diff. (And Cheerio is what's used extensively for the WCAG 2 build system)

In the case of a Note/etc. occurring first, the link is currently moved below the note. Using a contrived, locally-edited example:

This PR also restructures a bit of CSS so that we don't need a remark plugin solely for the purpose of adding a class to a bunch of child elements to increase their font size.

Note that this PR leaves a couple of functions unused in lib/markdown/guidelines.ts; I expect those to be useful again later (a further-future PR may move them).

FWIW, the diff of EntryText.astro might be slightly quieter with whitespace ignored (?w=1).

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