Play Framework

docs: OpenAPI generation + annotation reference (PF-12 / PF-81)

Add a fork-specific reference doc for the OpenAPI 3 spec generation that
shipped in PF-12 and the annotation-driven enrichment that shipped in
PF-81. Covers: the three exposed endpoints (json/yaml/docs), configuration
keys, what works without any annotations, the five Swagger annotations the
generator reads (@Operation, @ApiResponse(s), @RequestBody, @Parameter,
class-level @Tag), worked examples, deliberate scope cuts on @Schema field
coverage, and a CI export pattern via curl.

Lives at documentation/fork-features/openapi.textile — a new directory for
fork-specific detailed feature docs in the same textile format as the
upstream documentation/manual/, but kept separate per the rule that the
manual itself is preserved as-is. README gains a short summary section
linking out to the textile reference.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>