docs: add title and brief module description

docs/index.rst

 .. _swh-core:
 
-Software Heritage - Development Documentation
-=============================================
+Software Heritage - Core foundations
+====================================
+
+Low-level utilities and helpers used by almost all other modules in the stack.
+
 
 .. toctree::
    :maxdepth: 2
    :caption: Contents:
 
 
-
 Indices and tables
 ==================