Add an introduction on the “Getting started” cover page

docs/getting-started/index.rst

 Getting started
 ===============
 
+Software Heritage has developed an infrastructure to collect, preserve,
+analyze and present software in source code form.
+
+You can :doc:`learn how to use its public API <api>`. Instead of using
+the public archive, you might want to :ref:`use a local installation
+<getting-started>` while developing clients.
+
+Contributions to Software Heritage’s own source code
+are very much welcome. You will need to take the time to :ref:`setup a
+developer environment <developer-setup>` before you can start.
+
 .. this index file is used to force a hierarchy in the toctree,
    even though these two files are in the parent directory