Add documentation for command-line tools

docs/cli.rst

+Command-line interface
+======================
+
+Shared command-line interface
+-----------------------------
+
+.. click:: swh.core.cli:swh
+  :prog: swh
+  :show-nested:
+
+Database initialization utilities
+---------------------------------
+
+.. click:: swh.core.cli.db:init
+  :prog: swh db init
+  :show-nested:
+
+.. click:: swh.core.cli.db:db_init
+  :prog: swh db-init
+  :show-nested:

docs/index.rst

@@ -12,4 +12,5 @@ Reference Documentation
 .. toctree::
    :maxdepth: 2
 
+   cli
    /apidoc/swh.core