Document the /status health-check endpoint and its query options

Describe the problem to be solved

Grist exposes a health-check endpoint at GET /status with several useful query options:

• ?db=1 — also check home DB connectivity (runs SELECT 1)
• ?redis=1 — also check the Redis connection
• ?docWorkerRegistered=1 — also check doc worker registration in Redis
• ?ready=1 — also check that the server finished starting up
• ?timeout=<ms> — per-check timeout (default 10000)

It returns 200 with Grist <name> is alive (<check> ok, ...) when healthy, or 500 with ... is unhealthy (<check> not ok) otherwise.

These options (added in fbae81648) are only described in a code comment in FlexServer.ts; there is nothing about them in documentation/. Operators wanting a liveness/readiness probe that also verifies the database connection have no discoverable way to know GET /status?db=1 already does exactly that.

Describe the solution you would like

Add a short section to the documentation describing the /status endpoint, each query option, the timeout parameter, and the response format / status codes, so it can be used as a container/orchestrator health probe.