docs: split reference material into docs/, keeping the README a shop window (v0.23.4-beta)

Documentation only — no functional change.

The README was carrying two jobs for two different readers. Its Reference section was
the single largest block at 154 lines, and nobody deciding whether to try the project
reads an InfluxDB environment variable table — while the people who need it were
scrolling past the pitch to reach it.

Split along that seam: anything you would read before installing stays in the README,
anything you would read after installing moves to docs/.

  README.md              430 lines   sell, prove, install, feature overview
  docs/change-guard.md   193         safety system, settings, guard states
  docs/architecture.md   116         data stores, polling model, module layout
  docs/capsman.md         84         role model, radio matching, current limits
  docs/alerting.md        81         rules, channels, webhooks, maintenance windows
  docs/api.md             77         scoped tokens, preflight, scripting
  docs/configuration.md   76         environment, secrets, rotation, TLS
  docs/sso-oidc.md        71         identity provider setup and behaviour
  docs/README.md          20         index

In-repo rather than a documentation site, deliberately. A change to Change Guard's
semantics can edit its documentation in the same commit as the code — which, given how
often that behaviour has been refined recently, is the property that actually matters.
A static site generator can point at docs/ later without moving a file.

The Change Guard pitch stays on the front page: the problem framing, the verified
recovery timeline and the quoted prediction warning are what make someone care, and a
reference page is the wrong home for them.

Three sections are new rather than relocated, written because the split exposed the gaps:

  - architecture.md did not exist. It now records why there are three data stores, the
    polling cadences, and which modules are pure functions over a snapshot and why.
  - Scoped API tokens shipped without usage documentation. api.md shows preflighting a
    change from a script and reading the guard block afterwards.
  - change-guard.md records why ip-on-bridge-port is informational rather than critical
    on RouterOS 7 — reasoning that previously existed only in a commit message.

Every internal anchor and cross-file link verified to resolve, including anchors into
other documents. No content dropped, checked by keyword comparison against the previous
single file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/README.md

@@ -0,0 +1,20 @@
+# MikroTik Manager documentation
+
+Reference material for people already running the platform. If you are still deciding
+whether to try it, the [project README](../README.md) is the better starting point.
+
+| Document | Covers |
+|---|---|
+| [Configuration](configuration.md) | Environment variables, secret management and key rotation |
+| [Change Guard and Config Health](change-guard.md) | How the safety system works, its settings, and what to expect when it fires |
+| [Alerting](alerting.md) | Alert rules, delivery channels, webhooks, scheduled reports, maintenance windows |
+| [Single sign-on (OIDC)](sso-oidc.md) | Identity provider setup, group-to-role mapping, break-glass behaviour |
+| [CAPsMAN](capsman.md) | How centrally provisioned access points are modelled, and current limits |
+| [API and automation](api.md) | Scoped tokens, authentication, and driving the platform from scripts |
+| [Architecture](architecture.md) | How the system is put together, for contributors |
+
+## Conventions used here
+
+- **Settings in the UI** are stored in the database and apply without a restart.
+- **Environment variables** live in `.env` and require a container restart to change.
+- Anything described as *read-only* makes no writes to your devices.