Docs: guides in the indexes, the stalest-kind rule, and the changelog

Guides was shipped without an entry in PLUGINS.md or START-HERE, so a site
adopting the product had no way to find out it exists.

BACKUP-KINDS.md described a host's badge as coming from the most recently
confirmed kind. It has not worked that way since the fleet report was reworked:
the freshest revision per (host, kind) is taken, and then the STALEST of those
kinds decides, because reporting the freshest would let a healthy NTLARS backup
mask a part-marker capture that stopped a month ago. Stalest among kinds that
can be JUDGED - a kind never confirmed is unmeasurable, not stale, and treating
it as the worst value made a host with a working backup show no badge at all.

The backups row also still advertised UDC and CMM kinds that do not exist.

api-inventory, openapi.json and PROJECT-MAP regenerated.

docs/PROJECT-MAP.md

@@ -88,6 +88,6 @@ Manifest-less directories under `plugins/` are core frontend surface and always
 
 ## Size
 
-- test functions defined: **1305** (parametrised cases collect higher)
-- documented API paths: **285** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)
+- test functions defined: **1333** (parametrised cases collect higher)
+- documented API paths: **299** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)