Add reference modules to documentation (#9)

`mkdocs` picks up docstrings in Python modules to generate HTML
documentation.
Add all existing modules to the index.

docs/reference/cli.md

@@ -0,0 +1,3 @@
+# CLI
+
+::: dbt_score.cli

docs/reference/exceptions.md

@@ -0,0 +1,3 @@
+# Exceptions
+
+::: dbt_score.exceptions

docs/reference/models.md

@@ -0,0 +1,3 @@
+# Models
+
+::: dbt_score.models

docs/reference/rule.md

@@ -0,0 +1,3 @@
+# Rule
+
+::: dbt_score.rule

docs/reference/rule_registry.md

@@ -0,0 +1,3 @@
+# Rule registry
+
+::: dbt_score.rule_registry

docs/reference/rules/generic.md

@@ -0,0 +1,3 @@
+# Generic
+
+::: dbt_score.rules.generic

mkdocs.yml

@@ -6,3 +6,11 @@ plugins:
   - mkdocstrings
 nav:
   - Home: index.md
+  - Reference:
+      - reference/cli.md
+      - reference/exceptions.md
+      - reference/models.md
+      - reference/rule.md
+      - Rules:
+          - reference/rules/generic.md
+      - reference/rule_registry.md