Document buildings and levels where the old single-map model was still taught

The 0.11.0 release changed what a map position means and six documents still
described the model it replaced. Each of these could have caused a real mistake
rather than being merely out of date:

- IMPORT-API mapped legacy mapleft/maptop to mapx/mapy with no mention of the
  level, so a scripted import - including the classic-ASP one still to run
  against production - would have produced markers the map shows as "level
  unknown". It now maps levelid too and says how to resolve the default level.
- API-REFERENCE enumerates the unauthenticated surface in full, because that is
  what a deploy reviewer reads, and the three public /api/maplevels reads were
  missing from it. Also records why the write split is asymmetric: repositioning
  needs assets.edit, creating a level needs admin, since a level's dimensions are
  the coordinate space every marker on it is expressed in.
- CONFIG still presented the four map_* settings as live, telling the reader to
  re-upload a blueprint in a settings page that no longer drives the map. They
  are marked superseded and kept for downgrade.
- UPGRADE gained a 0.11.0 section: nothing moves on screen, and replacing a
  blueprint with one of different dimensions moves every marker on that level, so
  recalibrate from landmarks rather than editing width and height.
- PLUGIN-HOOKS now states that a map overlay keys on assetid and must not return
  coordinates or a level - a second copy of a position is one that can disagree.

Adds FLOOR-MAP.md, the operator's page: loading a plan, placing markers, and
what to do when the plan changes, with the reasoning left in ADR-017. START-HERE
routes to it from the new-site path, and specifically as the page to read BEFORE
a floor plan changes.

docs/PLUGIN-HOOKS.md

@@ -369,6 +369,12 @@ class MeasuringToolsPlugin(BasePlugin):
 Consumed by `GET /api/pluginui/map-overlays` (disabled plugins skipped; broken
 plugin isolated in prod, re-raised in dev/test).
 
+An overlay keys on `assetid` and never on coordinates, so it needs to know
+nothing about buildings or levels (ADR-017): it decorates whichever marker that
+asset already has, on whatever level that marker is on. Do not return `mapx` /
+`mapy` / `levelid` from an overlay endpoint - a position belongs to the asset,
+and a second copy of it in an overlay is one that can disagree.
+
 ### `get_asset_presentation() -> List[Dict]`
 
 Returns asset-type presentation/routing definitions. Added in contract 0.7.0