docs: Add process-based plugin docs (#1669)

Still need more, but this is a good first pass

Author: kyleconroy

docs/guides/plugins.md

@@ -0,0 +1,58 @@
+# Authoring plugins
+
+## Process plugins
+
+To use process-based plugins, you must be using [Version
+2](../reference/config.html) of the configuration file. The top-level `plugins`
+array defines the available plugins and maps them to an executable on the system.
+
+In the `codegen` section, the `out` field dictates what directory will contain
+the new files. The `plugin` key must reference a plugin defined in the
+top-level `plugins` map. The `options` are serialized to a string and passed on
+to the plugin itself.
+
+```json
+{
+  "version": "2",
+  "plugins": [
+    {
+      "name": "jsonb",
+      "process": {
+        "cmd": "sqlc-gen-json"
+      }
+    }
+  ],
+   "sql": [
+    {
+      "schema": "schema.sql",
+      "queries": "query.sql",
+      "engine": "postgresql",
+      "codegen": [
+        {
+          "out": "gen",
+          "plugin": "jsonb",
+          "options": {
+            "indent": "  ",
+            "filename": "codegen.json"
+          }
+        }
+      ]
+    }
+  ]
+}
+```
+
+For a complete working example see the following files:
+- [sqlc-gen-json](https://github.com/kyleconroy/sqlc/tree/main/cmd/sqlc-gen-json)
+  - A process-based plugin that serializes the CodeGenRequest to JSON
+- [process_plugin_sqlc_gen_json](https://github.com/kyleconroy/sqlc/tree/main/internal/endtoend/testdata/process_plugin_sqlc_gen_json)
+  - An example project showing how to use a process-based plugin
+
+### Security
+
+Process-based plugins offer minimal security. Only use plugins that you trust.
+Better yet, only use plugins that you've written yourself.
+
+## WASM plugins
+
+*Coming soon!*

docs/index.rst

@@ -74,4 +74,5 @@ code ever again.
    :hidden:
 
    guides/development.md
+   guides/plugins.md
    guides/privacy.md

docs/reference/config.md

@@ -120,6 +120,16 @@ The `gen` mapping supports the following keys:
   - Filename for the generated JSON document. Defaults to `codegen_request.json`.
 - `indent`:
   - Indent string to use in the JSON document. Defaults to `  `.
+
+### plugins
+
+Each mapping in the `plugins` collection has the following keys:
+
+- `name`:
+  - The name of this plugin. Required
+- `process`:
+  - `cmd`:
+    - The executable to call when using this plugin
   
 ## Version 1
 

docs/reference/language-support.rst

@@ -1,4 +1,4 @@
-Database and Language Support
+Database and language support
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