docs: Diátaxis documentation overhaul (compile-verified)

[QuitHub]

Radically expand and restructure the documentation (Diátaxis)

The published docs were 4 tutorial pages. This PR turns them into a full
Diátaxis documentation set — 45 pages across
Getting Started, Tutorials, How-to guides, Concepts and Reference — and fixes
the broken tutorial/intro.md stub.

What's new

• Getting Started: overview, installation, 5-minute quick start.
• Tutorials: the in-memory (Star Wars) and DB-backed (World) tutorials,
  revised with cross-links and a corrected schema; plus a new mutations &
  subscriptions tutorial.
• 14 how-to guides: filtering/ordering/paging, composing mappings, SQL
  backends, interfaces/unions, jsonb, custom scalars/enums, schema & query
  directives, effects/batching, errors, mapping validation, generic
  derivation, circe, serving over HTTP.
• 9 concept pages: architecture (compiler → algebra → interpreter),
  mappings & cursors, elaboration, schema model, nullability, effects,
  composition, introspection.
• 14 reference pages: schema/SDL, mapping types, query algebra, predicates,
  Result/Problem, cursor, context/env, elaboration phases, and the SQL/circe/
  generic backends.

Compile-verified

Every example is real, compiled code. Most are snipped from existing demo and
test mappings via the existing Output.snip mechanism — those sources gained
comment-only // #tag markers (no behaviour change). Three tiny in-memory
example mappings live in modules/docs; the docs module now depends on
core/circe/generic so those and the inline mdoc blocks compile.

sbt docs/tlSite builds clean (mdoc compilation + Laika strict internal-link
validation), and sbt rootJVM/Test/compile confirms the tagged sources still
compile.

Notes for reviewers

• Code-bearing example sources: modules/docs/src/main/scala/grackle/*.scala.
• All other example code is snipped verbatim from demo/ and modules/*/test.
• No library/runtime code changed; the only src/main edits are comment tags.