docs: add recovery guide and link it from README, quickstart, and troubleshooting (#1283)

Add a consolidated Recovery page that answers "what do I do if Sencho
breaks?" in one place. It covers getting back to a working state when
Sencho itself fails, a stack deploy fails, an admin is locked out of
sign-in, Docker is unavailable, or a remote node is unreachable, and
summarizes each path with a link to the authoritative detail page rather
than duplicating it.

The page opens by stating the core safety fact: Sencho keeps its state in
DATA_DIR and COMPOSE_DIR and treats compose files on disk as the source of
truth, so recovery is mostly file-level and running containers are
unaffected by a Sencho outage. It points to Backup & Restore as the
prerequisite.

Link the guide from the README documentation section, the quickstart
"where to next" cards, and the top of the troubleshooting page.

docs/docs.json

@@ -179,6 +179,7 @@
               "operations/self-hosting",
               "operations/upgrade",
               "operations/backup",
+              "operations/recovery",
               "operations/troubleshooting",
               "operations/verifying-images",
               "operations/trivy-setup",