Add a link to Mojo documentation main page to the document index

Change-Id: I7c6d15d75fd6ea4a936729bc2b07445778fae165
Reviewed-on: https://chromium-review.googlesource.com/c/1281867
Reviewed-by: Ken Rockot <[email protected]>
Commit-Queue: Oksana Zhuravlova <[email protected]>
Cr-Commit-Position: refs/heads/master@{#600057}
1 file changed