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]>

.gitignore

@@ -21,3 +21,5 @@ Thumbs.db
 .idea/
 .vscode/
 media_cache/
+site/
+/wiki/