Play Framework

docs: drop upstream-tracking framing, add CONTRIBUTING + CoC (release 1.13.54)

This project no longer tracks playframework/play1. Remove the upstream-tracking
machinery and the derivative framing from the user-facing docs.

- Delete sync-fork.sh. It was already dead: it rebased onto origin/master and
  pushed to a remote named `fork`, neither of which exists in this layout.
- README: retitle, replace the upstream marketing intro, and repoint the
  "Learn more" section, which sent readers to playframework.com/download and
  the playframework/play1 contributor wiki. Keep an Apache-2.0 attribution
  line in the Licence section.
- SECURITY: reword fork references and document that upstream advisories no
  longer reach this codebase automatically, in either direction.
- NOTICE: header retitled only. The derivation statement below it is
  load-bearing Apache-2.0 attribution and is deliberately unchanged.
- Add CONTRIBUTING.md, covering the build/test workflow, commit conventions,
  and the two silent-failure traps: a dependencies.yml bump goes green in CI
  because the build compiles against the committed framework/lib jars, and
  Tailwind CSS has no regeneration check at all.
- Add CODE_OF_CONDUCT.md (Contributor Covenant 2.1). Both new files are
  excluded from the distribution zip, which keeps README, SECURITY, LICENSE
  and NOTICE -- the docs that mean something to someone holding the artifact.
- Delete .github/ISSUE_TEMPLATE.md, inert because .github/ISSUE_TEMPLATE/
  shadows it, and repoint the remaining templates at the local docs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>