docs: SEO-tighten README intro, add FAQ, broaden npm description

The opening paragraph was a dense block of ten inline anchors, which is
what search engines pull for snippets. Rewritten as a lead sentence plus
prose, same information and same links retained.

Adds an FAQ section targeting the long-tail questions people actually
search (official API, reading chat in Node, tracking gifts/likes/viewers,
auth requirements, production use). Purely additive — no existing section
was moved or restructured.

npm description now covers likes, follows, viewers and 'real-time' rather
than only comments and gifts; it is the search snippet on npmjs.com and is
mirrored by Socket, Snyk, jsDelivr and libraries.io.

Also fixes a pre-existing broken anchor: the intro linked [Subscriptions]
(#subscribe), which has had no corresponding heading since the TS rewrite.