sphinx: add a general index
This index is automatically generated by Sphinx, and all terms from
the glossary are listed. It seems very convenient for the Yocto
Project documentation.
The implementation with an 'almost' empty file is tricky. It was
borrowed from:
https://stackoverflow.com/questions/40556423/how-can-i-link-the-generated-index-page-in-readthedocs-navigation-bar
(From yocto-docs rev: 8af595c464b58cf46df7ef067832db5c841e9202)
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Nicolas Dechesne