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rather than being merely out of date: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  are marked superseded and kept for downgrade.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

@@ -13,6 +13,9 @@ exists.
 2. [OPERATE-WINDOWS](OPERATE-WINDOWS.md) - restart, logs, backups, upgrades.
 3. [CSV-IMPORT](CSV-IMPORT.md) if the site's data is in spreadsheets, or
    [IMPORT-API](IMPORT-API.md) if there is a source database to script against.
+4. [FLOOR-MAP](FLOOR-MAP.md) - load the site's floor plans and place markers.
+   Also the page to read BEFORE a plan changes: it covers moving every marker
+   onto a redrawn plan without doing it one at a time.
 
 **Do not** follow [INSTALL-WINDOWS-IIS](INSTALL-WINDOWS-IIS.md) for a new site.
 That is the manual procedure, kept for hand-built servers that predate the