change docs index.rst

docs/source/index.rst

 
-Welcome to pystencils's documentation!
-======================================
+pystencils Development Branch Documentation
+===========================================
+
+These are the documentation pages for the development branch
+`backend-rework <https://i10git.cs.fau.de/pycodegen/pystencils/-/tree/backend-rework?ref_type=heads/>`_,
+which hosts the rework of pystencils' code generation backend.
+This is an ongoing project; as such, the documentation, too, is still under construction and bound to change rapidly.
 
 .. toctree::
    :maxdepth: 1