Move the bridge demo into its own document

[QuiteYellow]

First of the README split. The file was 1188 lines serving a library consumer, an appliance owner, a bridge deployer and a contributor at once, and pyproject sets readme = "README.md", so a pip install smartthings-local reader also got Unraid deployment steps.

docs/bridge-demo.md takes the bridge material: what it exposes, how it is configured and deployed, per-appliance coverage, config keys, MQTT topics, entity counts. 245 lines moved, README down to 944.

Two of those sections were in the wrong parent

"What the demo bridge gives you" and "Under the hood" were h3 children of "BLE OCF framing", so a codec section owned the bridge's architecture. That was the real reason the material was hard to find.

The judgement calls

• Per-appliance notes went with the bridge. The capability tables are written around the bridge's poll tiers and entity behaviour, so they read as bridge coverage. A library consumer wanting the same protocol facts has "Traps to avoid" and the API reference.
• Repo layout stayed, promoted to h2. It orients anyone in the repo, including the library tree, and its ## Reference parent left with the bridge config.
• Traps to avoid stayed. OBSERVE path selection, leaving /oic/sec/* alone, one session per peer: those are protocol lessons that outlive the demo.
• Parts 1 to 4 stopped being a sequence once 3 and 4 moved, so the numbering is gone from all four, and the three prose references to "Part 1" and "Part 2" now name their sections.
• The four library sections that were also h3 children of "BLE OCF framing" are reparented under a new Library reference h2.

A guard for the dependency this created

docs/bridge-demo.md now links back into the README, and the README links out to the docs. A cross-file anchor breaks silently: GitHub serves the page and does not scroll, so a renamed heading leaves no trace. A test walks the links between the README and every file in docs/, checks the target file exists, and checks the anchor matches a heading there. Confirmed by renaming a target heading and watching it fail.

What this did not fix

"BLE OCF framing" still owns 234 lines covering three library topics under one heading: the codec, the authentication providers (CertificateAuth, PskAuth, validate_identity) and OwnerPSK derivation. Authentication is the one thing every consumer needs, and it has only prose to find it by, which leaves it unlinkable and absent from the sidebar. Sorting that out is the library-reference pass, and its own change.

Validation

798 tests pass on Python 3.11, 3.12, 3.13 and 3.14, and on the dependency floor. check_share_safety.py is clean. Line accounting checks out: 245 lines left the README, and the new file is 258 including its own 13-line header.