Play Framework

docs: relabel manual as Play 1.12 + cross-cut restructure (PF-84 part 1)

PF-84 takes ownership of documentation/manual/ as the official Play 1.12
manual instead of treating it as upstream-preserved-as-is. This commit
covers the cross-cutting and structural pieces (the per-feature content
edits land in PF-84 parts 2 and 3 from sibling agents).

home.textile
- Top welcome line and release-notes link relabelled 1.5.x -> 1.12.x.
- TOC: new top-level entries for OpenAPI 3 (placed after HTTP routing),
  HTTP server / virtual threads / HTTP/2 / HTTP/3 (after Production
  deployment), and Observability (after Deployment options). Anchors
  match the predetermined filenames Agent 3 is creating.
- TOC: new sub-bullets under Logging configuration (Structured JSON
  logging), Configuration in several environments (Environment variable
  interpolation), Security Guide (Default security headers, CORS,
  SameSite cookies), and Deployment options (Behind a reverse proxy).
- Version notes: 1.12.x added at the top of the release-notes list;
  historical 1.5.x..1.0.x entries kept as legitimate references to past
  releases.

samples.textile rewritten
- The samples-and-tests/ directory was removed in cf494a7ba; the page
  now points readers at `play new`, `play new --frontend nuxt`, and the
  framework's own integration test fixture under
  framework/test-src/integration/testapp/.

deployment.textile rewritten
- WAR / JEE app servers / Google App Engine / Stax / CloudBees /
  Cloud Foundry / AWS Elastic Beanstalk / Heroku-specific sections
  removed. PF-78 dropped servlet support and most of the rest is long
  obsolete.
- Standalone deployment section freshened: play start / stop, log
  tailing, JVM tuning notes that reflect the virtual-thread baseline.
- New "Behind a reverse proxy" section covering nginx, HAProxy, AWS
  ALB/NLB, Cloudflare, with TLS-at-the-edge vs TLS-at-Play patterns,
  X-Forwarded-* config, http.path prefix rewriting, an nginx example,
  and WebSocket / SSE pass-through gotchas.
- Cross-link at the top to httpserver.textile (Agent 3's new file) for
  the embedded Netty server's protocol capabilities.
- Generic systemd unit replaces the old upstart snippet.

README.md
- OpenAPI link target updated from documentation/fork-features/openapi
  .textile to documentation/manual/openapi.textile (Agent 3 integrates
  the content into the manual proper as part of PF-84).

documentation/fork-features/openapi.textile deleted
- Content moves into documentation/manual/openapi.textile via Agent 3.
  The fork-features/ directory is deleted entirely.

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