docs: mention the npm dependency more clearly (Issue #345)

docs/overview.rst

@@ -141,9 +141,11 b' installed.'
   but build the Kallithea package itself locally instead of downloading it.)
 
 .. note::
-   Kallithea includes front-end code that needs to be processed first.
-   The tool npm_ is used to download external dependencies and orchestrate the
-   processing. The ``npm`` binary must thus be available.
+   Kallithea includes front-end code that needs to be processed to prepare
+   static files that can be served at run time and used on the client side. The
+   tool npm_ is used to download external dependencies and orchestrate the
+   processing. The ``npm`` binary must thus be available at install time but is
+   not used at run time.
 
 
 Web server

docs/setup.rst

@@ -54,7 +54,9 b' path to the root).'
           but when trying to do a push it will fail with permission
           denied errors unless it has write access.
 
-Finally, prepare the front-end by running::
+Finally, the front-end files must be prepared. This requires ``npm`` version 6
+or later, which needs ``node.js`` (version 12 or later). Prepare the front-end
+by running::
 
     kallithea-cli front-end-build