Add a public API wiki at /wiki

Builds docs/ into a static site via mkdocs + mkdocs-material in its own
Docker build stage (docs-builder), so mkdocs itself never ends up in the
runtime image -- only the rendered HTML does, mounted at /wiki alongside
the existing /docs OpenAPI schema (unconditionally, unlike /docs, since
this is curated content meant to be public regardless of
AURORA_API_DOCS_ENABLED).

docs/aurora-core-spec.md (the internal build spec, referenced by "section
N" comments throughout the codebase) is excluded from the built site
entirely via exclude_docs, not just left out of the nav -- it was never
meant for public consumption.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

docs/api.md

@@ -2,7 +2,8 @@
 
 Base path: `/api/v1`. Interactive OpenAPI docs are available at `/docs` in
 any environment where `AURORA_API_DOCS_ENABLED=true` (default; set to
-`false` to hide docs in production — section 32).
+`false` to hide docs in production — section 32). This same document is
+also published as a browsable page at `/wiki/api/` (see `mkdocs.yml`).
 
 All responses are JSON, built from Pydantic response schemas
 (`app/schemas/`), never from serialized ORM objects (section 18). Errors use