Split the README into docs/ pages, add web UI screenshots

- README is the overview; web UI, pages, Home Assistant, how it works,
  wiring, Nano over WiFi and the recovery app have their own page in docs/.
- docs/web-ui.md has a screenshot of every tab, made with headless Edge by
  tools/make_webui_screenshots.py.
- The web UI opens a page in the editor with #pages/<name>. Version 2.7.2.
- The reply wire is fitted (flashing the Nano over WiFi works).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/README.md

@@ -5,7 +5,7 @@ PlatformIO environments: `esp32` (OTA) and `esp32_usb` (USB). Tests: `test_esp32
 The ESP32 decides what is shown on the display. It never touches the panel itself. The
 BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent to the Nano over
 `Serial2` (UART2, GPIO 17 TX / GPIO 16 RX, 115200 baud). `Serial` (USB) is only used for
-debug output. Wiring is described in the [main README](../../README.md#wiring-between-esp32-and-nano).
+debug output. Wiring is described in the [documentation](../../docs/wiring.md).
 
 ## Files
 
@@ -53,7 +53,7 @@ debug output. Wiring is described in the [main README](../../README.md#wiring-be
 | `net/wifi_ota.*` | WiFi (with a restart when it stays down), ESP32 OTA updates, OTA-only mode |
 | `net/home_assistant.*` | MQTT and the Home Assistant entities (power switch, mode and off image dropdowns, IP and firmware sensors, recovery button) |
 | `net/ha_api.*` | Reads Home Assistant entity values over the REST API for the dynamic pages |
-| **`web/`** | **Web UI** (see the [main README](../../README.md#web-ui)) |
+| **`web/`** | **Web UI** (see the [documentation](../../docs/web-ui.md)) |
 | `web/web_ui.*` | Web server and JSON API (the endpoints are listed in `web_ui.h`) |
 | `web/page_json.*` | Dynamic pages as JSON |
 | `web/schedule_json.*` | The page schedule and rules as JSON |
@@ -73,7 +73,7 @@ again. Several layers take care of that:
 | A new OTA firmware crashes, hangs or can't connect to WiFi | **Rollback.** A new firmware is only marked as good after it has run for 60 s with WiFi. If it restarts before that, the bootloader starts the previous firmware again. |
 | The firmware hangs in `loop()` | **Watchdog.** If `loop()` doesn't run for 30 s, the ESP32 restarts. That counts as a crash. |
 | The firmware crashes once or twice | **OTA window.** After a crash the ESP32 runs only WiFi and OTA for 15 s (LED blinks fast), then starts the main code again. |
-| The firmware keeps crashing (3 times in a row) | **Recovery app** for 10 minutes (see the [main README](../../README.md#recovery-app)). Then the main code gets one more try; one more crash brings the recovery app back. Running 5 minutes without a crash clears the counter. Without the recovery app installed: only WiFi and OTA run, for 10 minutes. |
+| The firmware keeps crashing (3 times in a row) | **Recovery app** for 10 minutes (see the [documentation](../../docs/recovery.md)). Then the main code gets one more try; one more crash brings the recovery app back. Running 5 minutes without a crash clears the counter. Without the recovery app installed: only WiFi and OTA run, for 10 minutes. |
 | WiFi doesn't connect at boot (30 s), or stays down for 10 minutes | **Recovery app**, which keeps trying and opens the **FlipDot-Setup** WiFi with a setup page to choose another WiFi. Without the recovery app: restart. |
 | The firmware runs, but its OTA is broken | **Recovery app** through the Home Assistant button, or by switching the power on and off 3 times quickly. |
 
@@ -102,7 +102,7 @@ Rules for changing the code:
 - **Status on the display:** `Connecting to WiFi...` and `Getting the time...` at boot,
   `Crashed!` messages in the OTA-only modes, and `No WiFi` with the time when WiFi is down
   for more than a minute. See the table in the
-  [main README](../../README.md#recovery-app).
+  [documentation](../../docs/recovery.md).
 
 - **Frames only when something changes:** a new frame is sent when the minute changes, the
   screen changes (clock, page, off image) or the Nano was just powered on. The serial line is
@@ -134,7 +134,7 @@ Rules for changing the code:
     idle UART line is high and would otherwise power the Nano through its RX pin.
 - **Nano flash bridge:** `pio run -e nano_ota -t upload` flashes the Nano through the ESP32.
   While a flash session runs, the clock is paused. See the
-  [main README](../../README.md#flashing-the-nano-over-wifi).
+  [documentation](../../docs/nano-ota.md).
 - **Status LED** (built-in): off while the display is off, slow blink with WiFi, fast blink
   without WiFi.