docs: introduce per-package documentation

Documentation for packages should be placed in docs/packages/
and be added to docs/index.rst.

docs/index.rst

@@ -43,6 +43,12 @@ Developer Documentation
    dev/wan
    dev/i18n
 
+Packages
+--------
+
+.. toctree::
+   :maxdepth: 1
+
 Releases
 --------