docs: add brooms and mounts, races and Quidditch guides

[Kheartz]

Second of three stacked PRs. #12 has merged, so this is now based on main and its diff is this section alone. #14 is stacked on it.

Adds a Mounts and races section (sidebar orders 80–89), written for 1.8.2 and checked against the v1.8.2 Mod source:

• Brooms and mounts
  • All riding is client-side (Broom, FlyingMount, GroundMount, local player only) and shows to other players without a script. The server decides what each player owns through their inventory.
  • Lists the broom and creature item ids. The first broom or creature of each kind auto-equips.
  • No events fire for mounting, and mount() only means "accepted", so the sample retries until mountSettled.
  • Covers the broom overrides. They are per client rather than per resource, so the guide says to call resetOverrides(), and that teleport without a yaw faces 0.
  • Ends with an arena pattern: the server removes the items, and the client dismounts the rider and shuts the tool wheel.
• Races
  • Uses the experimental Races API, following the bundled races-demo design. Each racer's client runs its own copy of a stock race (BRR_01) on the same custom track: setTrack, setLeash(false), fitBounds, start, then boosts.
  • The server times each racer from the gameEvent reports and de-duplicates them.
  • Clean-up is per client.
• Quidditch: the props kit (ball, hoops, puppet bludgers, the self-flying snitch, banners). It detects nothing and each client spawns its own copy, so the server runs the match. clearAll() cleans up.

Checks: check_samples (111 samples, 1.8.2 contract), docs.mjs build, check_links and git diff --check all pass.