fix(docs): restore index.rst to fix RTD build (#46)

sphinx_gallery hardcodes index.html lookup in embed_code_links.
Using root_doc='readme_link' caused FileNotFoundError on RTD
clean builds (masked locally by stale shelve cache). Move
README include and toctree from readme_link.rst back to
index.rst and remove root_doc override.

Author: Caparrini

docs/conf.py

@@ -20,7 +20,6 @@
 copyright = '2024, Antonio Caparrini, Javier Arroyo'
 author = 'Antonio Caparrini, Javier Arroyo'
 release = version
-root_doc = 'readme_link'
 
 # -- General configuration ---------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#general-configuration