chore: rework plugin readmes, json changelogs and standalone repo setup

plugins/session-sharing/CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing to Session Sharing
+
+## Development
+
+```bash
+npm run build      # build into dist/
+npm run test       # run this plugin's tests
+npm run typecheck  # type-check this plugin
+npm run validate   # check manifest.json
+npm run format     # format the code with Prettier
+```
+
+## Settings
+
+### Admin
+
+- **Allow Session Sharing:** turn sharing on or off for everyone. When off, it overrides every host setting
+
+### Host
+
+- **Allow Session Sharing:** allow sharing sessions on this host
+
+## Permissions
+
+- `session-sharing.use`: share sessions and create and join meeting rooms. Admins and users have it by default.