docs: add settable properties tables to plugin docs

Renames the existing `## Internal properties` umbrella section to `## Properties` across all plugin docs and adds a new `### Settable properties` subsection enumerating every property accepted by each plugin's `:set` command. Each row lists scope, default, the actual `:report` flag shapes that surface the value, and a one-line description. The buildpacks plugin is included even though it uses `:set-property`, with a note in the subsection clarifying the legacy command name. Property lists, scopes, and report flags were verified against plugin source and the live dokku binary.

docs/appendices/file-formats/app-json.md

@@ -288,3 +288,11 @@ An autoscaling trigger consists of the following properties:
     - Example use-cases
         - Setting up OAuth clients and DNS
         - Loading seed/test data into the app’s test database
+
+## Properties
+
+### Settable properties
+
+| Property | Scope | Default | Report flags | Description |
+|---|---|---|---|---|
+| `appjson-path` | app + global | `app.json` | `--app-json-appjson-path`, `--app-json-global-appjson-path`, `--app-json-computed-appjson-path` | Path within the app to the `app.json` manifest, relative to the build root |