Update index.html from http://neovim.org/doc_index.

See also neovim/neovim.github.io#62.
Merge pull request #6 from fwalch/index-page

Update index.html from http://neovim.org/doc_index.
Generate badge for clang analysis
Merge pull request #7 from Hinidu/clang-badge

Generate badge for clang analysis
Generate user documentation.
Use Neovim green for captions.