Add Sizzle wiki as API page

[mgol]

The main page should document jQuery.expr.pseudos but we should also have shorter pages for jQuery.expr[ ":" ] & jQuery.expr.filters as its deprecated aliases.

Sizzle docs can be a starting point as jQuery.expr is an alias of Sizzle.selectors in jQuery <3.7.0:
https://github.com/jquery/sizzle/wiki#sizzleselectorspseudosname--function-elem--

[timmywil]

I don't know if it should. While the support probably can't go away, custom pseudos aren't necessarily something I'd want to encourage. It's better to use .filter() after selection.

[mgol]

OK, let's discuss further at the team meeting. 🙂 cc @gibson042

[timmywil]

We're going to change this issue to include the Sizzle wiki as its own API page, rather than document everything separately.