Embed demo video as iframe, link it from P5js, document endpoint JSON shapes

[davidchatting-bot]

Follow-up to #76 (merged) - this was pushed to the same branch just after that PR was merged, so it needs its own PR.

• Swapped the clickable-thumbnail video link for the actual YouTube `<iframe>` embed code. Note: GitHub's README renderer strips `<iframe>` tags for security, so this likely won't actually play inline on github.com itself - if that matters, the thumbnail-link version is the safer bet for guaranteed display there.
• P5js section now links `demo video` directly instead of saying "see above."
• Added a subsection per built-in endpoint with example request/response JSON, including details not obvious from the table alone (e.g. `password` is starred-out-same-length rather than omitted, `lastnetwork`/`LOCALHOST`/`GATEWAY` are omitted-when-false rather than `false`, `/yoyo/clients` returns `[]` outside peer-server mode).

README only, no code changes.