docs: fold the root docs/ tree into the Docusaurus site and delete it

docs/ was not the documentation site; it was a grab bag of long-form
design notes, wire contracts and observability guides that landed with
feature PRs because their authors needed somewhere to put them. Root
AGENTS.md already says long-form dev docs live in
website/docs/developer-guide/; this moves the 14 living documents there
(or to the matching user-guide section) so they are published, searchable
and linked from the sidebar instead of being found by grep only.

Developer guide: micro-compaction, gateway-session-lifecycle (was
session-lifecycle), state-db-recovery, multiplexing-gateway,
chronos-managed-cron-contract, relay-connector-contract, observer-hooks
(was observability/README), gateway-monitoring (observability/monitoring),
relay-shared-metrics, middleware, streaming-tts, billing-lifecycle.
User guide: egress/network-isolation (was security/network-egress-
isolation), features/kanban-multi-gateway (was kanban/multi-gateway).

Each page got title/description frontmatter and a sidebar entry; repo-
relative links became site links or GitHub blob URLs; two MDX brace
hazards escaped. Every in-tree pointer (module docstrings, config
comments, the relay conformance test's Path, the monitoring-doc test,
gateway-internals, cron-internals, kanban docs, .dockerignore, AGENTS.md)
now names the new location. `docusaurus build` passes with no unresolved
links on the moved pages.

AGENTS.md

@@ -252,7 +252,7 @@ families: `hermes_state.py` (21), `gateway/run.py` (15), `tools/mcp_tool.py` (15
   (`_SLASH_DISPATCH` in `cli.py`, `_command_handler_table` in the gateway are the shape).
 - **No re-export shims for internal moves** ("keep the old name importable"). Internal paths
   are not API; external compat is handled ONCE by the compat layer, not per PR.
-- **Moving a symbol means fixing its docs in the same PR:** grep `website/docs`, `docs/`,
+- **Moving a symbol means fixing its docs in the same PR:** grep `website/docs`,
   `skills/`, and every `AGENTS.md` for the old `path.py` + symbol (23 doc files went stale
   after the refactor). `evals/codebase_navigability/static_metrics.py <tree> <label>` measures
   file/function/CC/elif distributions before/after a large PR in ~2 min.