docs(api): Channels API, and once-per-window call limits on zones and scopes

[MrAlders0n]

Docs for the new public API call limits and the new Channels API.

Don't merge yet. This documents behaviour that isn't on the server yet. Merge once the server change is pulled, fanned out and the two new fail2ban jails are in (spec: MeshMapper_Server docs/SPEC-api-call-limits.md, e96028f).

What changes:

• new channels-api.md (get_channels.php on every region site: the region's hashtag channels with their key and hash), added to the nav after Scopes API
• zones-api.md: one call per country (get_zones) and per region site (get_geojson) every 23.5 h per IP. A second call inside the window is a 429 and a site-wide ban, 1 day doubling up to 30 days
• scopes-api.md: same idea, one call per region site every 55 min, ban 1 h doubling up to a week
• all three: new Call limits section, only a 200/304 uses the call, only GET counts, max-age now matches the window, 405 row, 429 body carries retry_after

The key/hash example values on the channels page are real (#ottawa = 7871ec72b45617696c35c970bddd8124, hash 21), checked in both PHP and Python.