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rather than being merely out of date: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  are marked superseded and kept for downgrade.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/CONFIG.md

@@ -196,12 +196,21 @@ The batch generator at `/print/asset-label-batch/<assettype>` (reached from the
 
 ### map
 
+**SUPERSEDED as of 0.11.0.** A blueprint and its pixel dimensions are properties
+of a LEVEL now, not of the site (ADR-017), because a site can have more than one
+building and a building more than one floor. Manage them in
+**Settings > Buildings and levels**, or through `/api/maplevels`.
+
+These four keys are still present so that downgrading finds the blueprint it had,
+and the levels migration copied their values onto the first level. Editing them
+changes nothing on the map.
+
 | Key | Default | Notes |
 |-----|---------|-------|
-| `map_blueprint_light` | `/static/images/floorplan-placeholder.svg` | Floor-map blueprint (light theme). Re-upload your own in Settings > Floor Map. |
-| `map_blueprint_dark` | `/static/images/floorplan-placeholder.svg` | Floor-map blueprint (dark theme). |
-| `map_width` | `3300` | Blueprint native width in pixels. |
-| `map_height` | `2550` | Blueprint native height in pixels. |
+| `map_blueprint_light` | `/static/images/floorplan-placeholder.svg` | Superseded by `maplevels.blueprintlight`. Retained for downgrade only. |
+| `map_blueprint_dark` | `/static/images/floorplan-placeholder.svg` | Superseded by `maplevels.blueprintdark`. |
+| `map_width` | `3300` | Superseded by `maplevels.mapwidth`. |
+| `map_height` | `2550` | Superseded by `maplevels.mapheight`. |
 
 ### integrations