[#736,doc][m]: upgrade to new okfn sphinx theme and other misc improvements.

* Edit on github link using edit on github extension
* Sidebar with local toc and global toc
* upgrade to sphinx 1.2b as we need later jquery for okf panel
[doc][s]: improve content of main page (shorter).
Merge pull request #757 from openspending/736-consolidate-docs

[#736]: upgrade to latest okfn sphinx theme.