Meta: update outdated parts of the FAQ

[domenic]

• Remove advice on using <h1>, per 6682bdeee6fb08f5972bea92064fe250f1b4ec9c.

• Update the mention of W3C forks, since all known ones redirect nowadays.

• Update "WHATWG and the W3C HTML WG" section to reflect the Memorandum of Understanding, and moved it toward the bottom since it's less important these days.