add core and per-plugin documentation set

docs/plugins/README.md

@@ -0,0 +1,20 @@
+# Plugin Doku Index
+
+- `rms-auth-otp-email.md`
+- `rms-auth-oauth.md`
+- `rms-auth-smtp-relay.md`
+- `rms-microham.md`
+- `rms-tx-audio-core.md`
+- `rms-debug-remote.md`
+- `rms-openwebrx-guard.md`
+- `rms-openwebrx-bandmap.md`
+- `rms-station-access-policy.md`
+- `rms-help-basic.md`
+- `rms-tx-control-native.md`
+- `rms-tx-state-file.md`
+- `rms-rfroute-shell.md`
+- `rms-rotor-hamlib.md`
+- `rms-vswr-native.md`
+- `rms-vswr-report-reader.md`
+- `rms-vswr-nanovna.md`
+- `rms-station-shell.md`