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>

documentation/genindex.rst

@@ -0,0 +1,3 @@
+=====
+Index
+=====

documentation/index.rst

@@ -22,3 +22,4 @@ Welcome to The Yocto Project's documentation!
    sdk-manual/sdk-manual
    toaster-manual/toaster-manual
    test-manual/test-manual
+   genindex