doc(NoCloud): Categorize the different configuration types (canonical…

…#5521)

Formally document providing runtime configuration in system configuration.
Introduce names to identify previously unnamed NoCloud concepts.
Add more structure - discrete sections for:
- runtime configuration types
- discovery configuration
- configuration sources

doc/examples/cloud-config-datasources.txt

@@ -38,7 +38,7 @@ datasource:
     # right here
     user-data: |
       # This is the user-data verbatim
-    meta-data:
+    meta-data: |
       instance-id: i-87018aed
       local-hostname: myhost.internal