Rewrite the README as highlights with screenshots; move the detail into docs/

The README was 1,240 lines of reference material and still described a
SQLite quick start. It is now a short tour (highlights, screenshots of the
web UI, an accurate quick start against Postgres), and each topic has its
own page under docs/ with an index: deployment, configuration, Alertmanager,
incidents, notifications, escalation, dead man's switches, single sign-on,
web UI, API and development. SERVICE-ACCOUNTS.md is rewritten from a
proposal into a reference, and TEAM-LOOKUP.md is gone with the endpoint it
described. The "Upgrading to ..." sections for an unreleased product are
dropped.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

docs/README.md

@@ -0,0 +1,21 @@
+# Terminal Duty documentation
+
+The [README](../README.md) is the short tour. These pages hold the detail.
+
+**Running it**
+- [Deployment](./deployment.md): Docker, the Helm chart, the database, backups, and the operator.
+- [Configuration](./configuration.md): environment variables and settings.
+- [Single sign-on](./single-sign-on.md): OIDC, group mapping, the terminal device flow.
+
+**Using it**
+- [The web UI](./web-ui.md): sessions, the Team and Admin tabs.
+- [Alertmanager configuration](./alertmanager.md): routes, integration keys and webhooks.
+- [Alerts and incidents](./incidents.md): correlation, lifecycle, on-call assignment, stale-alert expiry.
+- [Push notifications](./notifications.md): ntfy pages and acknowledging from them.
+- [Escalation](./escalation.md): ladders, repeats and the fallback topic.
+- [Dead man's switches](./dead-mans-switch.md): noticing that alerts stopped arriving.
+
+**Integrating and contributing**
+- [API reference](./api.md): every endpoint, authentication and error shape.
+- [Service accounts](../SERVICE-ACCOUNTS.md): non-human credentials for automation.
+- [Development and releasing](./development.md): tests, the CI gate, the release pipeline.