[doc] fix contributing link

Closes #38.

Also adds a caption to the toctree.
5 jobs for <span class="ref-name">doc-contrib-fix</span> in 4 minutes and 25 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #36740
integration
checkpatch

00:00:06

passed #36843
integration
repoquality

00:00:11

failed #36739
integration
repoquality

00:00:00

 
  Build
passed #36741
make:generic

00:01:40

passed #36742
knl
make:knl

00:02:27