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

plugins/step-ca/CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing to Step CA
+
+## 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
+
+- **CA URL, Root fingerprint and OIDC provisioner name:** leave all three empty to turn Step CA off
+- **Allowed private Step CA hosts:** the CA and, if it is internal, the identity provider. Private hosts not on this list are refused
+- **Redirect URI:** register `<base URL>/plugin-api/step-ca/callback` with your identity provider
+- **Use the old redirect URI:** keep sending the 2.8 URI `<base URL>/host/step-ca-callback`. Upgraded installs keep this on until you turn it off