U - Fix CSS linter errors: data.examples/expected-heading

[wbamberg]

As per its spec, the linter will check that pages which are expected to have an H2 "Examples" section actually have one.

Currently we have 89 "data.examples/expected-heading" errors. They are listed in the spreadsheet, all together in the "data.examples/expected-heading" tab.

This might be because those pages don't actually have examples, and it might be because those pages do have examples, but are including them under an H2 called "Example", which the linter will not notice.

Actually, though, I've just been through fixing up section headings here, so we can be pretty confident that these 89 pages are in fact in need of examples. That makes this a big user story, since writing 89 good examples is a substantial amount of work.

Examples ought to follow the linter's spec, and should ideally follow the convention described in #3139 (comment).

Acceptance criteria

• There are zero "data.examples/expected-heading" errors reported by the linter, when run against https://developer.mozilla.org/en-US/docs/Web/CSS