Part of #7898.

See also #3923 for some background here.

There are three cases:

1. seoSummary or summary are used to mark up the first non-macro paragraph. In these cases the classes are effectively a no-op and can be removed without anything changing. We can also automate these cases.

2. seoSummary or summary are used to truncate the first non-macro paragraph (usually taking the first sentence). In these cases it's probably worth checking that the whole first paragraph is reasonable to use as the summary, or whether we should split it. We don't have very precise guidelines for how long this should be though, and given that seoSummary and summary are usually omitted, I think it's OK to be pretty forgiving here.

3. seoSummary or summary are used to mark up a completely different part of the doc. In this case we should make sure that the first paragraph provides an appropriate summary. https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API/Using_AudioWorklet is a good example of a page that uses these classes intentionally, to provide an indirect opening paragraph with a direct summary. We'll need to stop doing things like this.

As far as I can tell the breakdown for Web/API is:

4072 pages - no summary or seoSummary
1423 pages - summary or seoSummary matches the first paragraph (case 1)
345 pages - summary or seoSummary truncates the first paragraph (case 2)
86 pages - summary or seoSummary selects a different text (case 3)