docs(mail_intake): add MkDocs navigation and API reference document structure

- docs(mkdocs.yml): define navigation structure for mail_intake modules
- docs(mkdocs.yml): configure mkdocstrings for Python API rendering
- docs(docs): add documentation root and mail_intake index page
- docs(docs): add adapters documentation pages with mkdocstrings directives
- docs(docs): add auth documentation pages with mkdocstrings directives
- docs(docs): add ingestion documentation pages with mkdocstrings directives
- docs(docs): add models documentation pages with mkdocstrings directives
- docs(docs): add parsers documentation pages with mkdocstrings directives
- docs(docs): add config and exceptions documentation pages

docs/mail_intake/parsers/subject.md

@@ -0,0 +1,3 @@
+# Subject
+
+::: mail_intake.parsers.subject