docs: reorganise documentation; derive header version from package.json; release v5.7.2

No protocol or message-protection changes.

The version in the application header was a literal and had fallen behind,
showing v5.6.0 while running 5.7.1. It now comes from package.json, and a test
fails if a hard-coded one reappears or if meta.json, the README badge, the
changelog and the docs disagree about the release.

Documentation reorganised so that everything technical lives in doc/ with an
index, and the root keeps only what belongs there by convention: README,
SECURITY, CHANGELOG and LICENSE.

- SECURITY.md rewritten. It listed a supported release line three major versions
  out of date and made claims the software does not make. It now states what is
  guaranteed, what is not, and how to report a problem.
- SECURITY_DISCLAIMER.md and RESPONSIBLE_USE.md merged into doc/USE-POLICY.md,
  which says what the software cannot protect against rather than listing
  generic advice.
- doc/SECURITY-ARCHITECTURE.md renamed to doc/ARCHITECTURE.md and rewritten
  around the session lifecycle, what verification gates, and how recovery works.
- doc/CRYPTOGRAPHY.md rewritten: key schedule, the Double Ratchet, framing, and
  memory handling, with values taken from the source rather than restated.
- doc/CONFIGURATION.md rewritten with the real file-type policy, ICE and TURN
  guidance, and the deployment caching rules that matter.
- docs/webrtc-config.md moved to doc/CALLS.md and rewritten; the obsolete
  docs/webrtc-audit.md, a working document full of stale line numbers, removed
  along with the docs/ directory.
- doc/CONTRIBUTING.md records what the recent regressions taught us about
  writing tests that can actually fail.
- doc/README.md added as an index.

Internal security review notes are excluded from the repository via .gitignore.
Those describe attack paths against specific releases in enough detail to
reproduce them, which is useful privately and harmful in public while users are
still updating.

doc/README.md

@@ -0,0 +1,25 @@
+# Documentation
+
+Technical documentation for SecureBit.chat. Start with the project [README](../README.md)
+if you are looking for an overview or a quick start.
+
+| Document | What it covers |
+| --- | --- |
+| [ARCHITECTURE.md](ARCHITECTURE.md) | How a session is established, verified and torn down, and where each guarantee comes from |
+| [CRYPTOGRAPHY.md](CRYPTOGRAPHY.md) | Key schedule, the Double Ratchet, SAS verification, memory handling |
+| [CONFIGURATION.md](CONFIGURATION.md) | Deployment, ICE and TURN setup, privacy modes, file transfer policy |
+| [CALLS.md](CALLS.md) | Voice and video: codec choices, adaptation, and why each value was picked |
+| [API.md](API.md) | Internal interfaces of the WebRTC manager and file transfer system |
+| [CONTRIBUTING.md](CONTRIBUTING.md) | Development workflow and what needs extra review |
+| [USE-POLICY.md](USE-POLICY.md) | Terms of use, intended use, and the limits of what the software can protect |
+
+Security policy and vulnerability reporting live in [SECURITY.md](../SECURITY.md) at
+the repository root, where GitHub expects to find them.
+
+## Keeping this accurate
+
+These documents describe the current release and are expected to change with it.
+Every value quoted here (limits, timeouts, algorithm parameters) is taken from the
+source rather than restated from memory, so if you change one in code, change it
+here in the same commit. [CONTRIBUTING.md](CONTRIBUTING.md) lists which documents
+are affected by which areas of the code.