deploy: add COPY-into-image tabs, clarify volume ownership, drop mounts

- deploy.md: new 'Getting your files into the image' subsection — a
  multi-tab annotated block (single file / directory / deps-first) showing
  COPY <src> <dest> patterns, plus a warning not to COPY secrets.
- clarify volume ownership: 'Logs and config are ours; the rest is yours' —
  we inject/manage the logs + config host mounts; devs put everything else
  in named volumes Docker owns.
- remove all /srv/mounts: dropped the MOUNTS_DIR volume + env var and every
  mounts reference from the compose block, injected-vars, paths line, and
  checklist. environments.md + index.md mounts mentions cleaned too.

Verified in-browser: COPY tabs switch, volume tip renders, zero mounts refs
remain; mkdocs build --strict clean.

Signed-off-by: disqualifier <dev@disqualifier.me>

docs/index.md

@@ -70,7 +70,7 @@ out of it; examples use placeholders like `<workspace>`, `<project>`, and
 
     ---
 
-    How to get a project running on **rethink-net** — containers, paths and
-    mounts, permissions, and secrets.
+    How to get a project running on **rethink-net** — the compose convention,
+    the one-command deploy, and secrets.
 
 </div>