docs: document the per-provider iframe URL strategy abstraction

Documents the new PuzzleProvider/discriminated-union
PuzzleIframeConfig shape, the src/lib/puzzleIframeUrl.ts strategy
module and why it replaces the previous provider-blind design, that
darkMode is now sent to AmuseLabs as a literal darkMode=0|1 query
param (in addition to the generic guardian-puzzle-context blob), and
that uid/darkMode are AmuseLabs-specific, not sent to Wordiply.
Updates "how to add a new puzzle type" to distinguish adding another
AmuseLabs-hosted game (just needs a set value) from adding a
genuinely new provider (needs its own config variant and builder
function). Updates remaining references to the old iframe.url/
hardcoded-URL wording to match the new registry shape.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: andresilva-guardian