docs: restructure the README and split the reference into docs/

[ogorzalka]

Stacked on #11.

The README had grown to ~1900 lines, was centred on PostQuery and was partly stale. It is now a short overview, with the reference split into pages under docs/:

• README.md: installation, quick start, the query classes, index
• docs/concepts.md (start here), results.md, upgrading.md
• docs/post-query.md, term-query.md, user-query.md, comment-query.md, site-network-query.md
• docs/tax-query.md, meta-query.md, date-query.md

How it was checked

• Every example's output is generated by running the code, not written by hand (72 examples).
• Every public method of the 13 classes appears in the docs; every method called in a snippet exists; every internal link and anchor resolves.
• The method → WordPress argument tables were checked by calling each method (194 pairs).
• The "before" column of upgrading.md was checked against the 1.0.3 tag.
• Statements about WordPress behaviour (LIKE wrapping, search wildcards, offset needing a limit...) were checked in the WordPress source.

Two code fixes found while documenting (separate commit)

• PostQuery::orderBy() sent any direction as is; it now throws a QueryException like the other classes.
• Search docblocks: users need a * for a partial match, sites and networks always match 'contains'.

Pest: 242 passed.