docs(faces): link the face-recognition guidance from where people look (#1125)

Every other feature routes readers to docs.picpeak.app. Face recognition was the
one that either pointed somewhere else or pointed at nothing — poor placement
for the feature with the highest read-before-you-enable burden anything here
ships.

.env.example referenced docs/feature-face-recognition.md, which does not exist —
and creating it is not the fix, because .gitignore:89 ignores docs/feature-*.md
outright, so the file would be invisible to anyone who cloned. That was the only
pointer to legal guidance an operator got while editing the variables that turn
Art. 9 processing on.

Also: the README linked the sidecar's developer README for the feature name and
had no row in the documentation table, docs/single-container.md left readers who
wanted the feature nowhere to go, ml/README.md had no backlink, and the admin
consent callout had no link at all. It does now, inline at the end of the
obligation.

Reported by @Luca-Timo.

.env.example

@@ -249,7 +249,8 @@ LOGS=./logs
 #
 # Face embeddings are biometric data (GDPR Art. 9 special category in the EU).
 # The photographer is the controller and needs a lawful basis for the people
-# in their photos — see docs/feature-face-recognition.md before enabling.
+# in their photos — read https://docs.picpeak.app/features/face-recognition
+# before enabling.
 #
 # NOT AVAILABLE ON THE ALL-IN-ONE IMAGE. The single-container build sets
 # PICPEAK_SINGLE_CONTAINER=true and the backend refuses to enable face