Restructure repo as monorepo: firmware/ + server/ + docs/

Moves the existing ESP-IDF captive_portal example into firmware/ to make
room for the new FastAPI server and project docs, ahead of building out the
full ESPresso Frame project (ESP32-C6 + Immich-backed e-ink photo frame).

.gitignore

@@ -0,0 +1,20 @@
+# ESP-IDF firmware build output
+firmware/build/
+firmware/sdkconfig
+firmware/sdkconfig.old
+firmware/managed_components/
+firmware/dependencies.lock
+
+# Python server
+server/__pycache__/
+server/**/__pycache__/
+server/.venv/
+server/*.egg-info/
+server/data/
+.env
+
+# Editor / OS cruft
+.vscode/
+.idea/
+*.swp
+.DS_Store