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.

ml/README.md

@@ -1,5 +1,10 @@
 # picpeak-ml
 
+> **Looking for how to use the feature?** →
+> [docs.picpeak.app/features/face-recognition](https://docs.picpeak.app/features/face-recognition)
+> covers enabling it, the per-gallery toggle, consent and what guests see.
+> This document is the sidecar's HTTP contract and deployment notes.
+
 Optional face-detection sidecar for PicPeak (#1074). Detects faces in one
 image and returns a bounding box, five landmarks, quality signals and a
 512-d embedding per face.