[RFC] Prepare doc_index.html, add canonical URLs.

Using a canonical URL will help if a search engine ever discovers /doc_index. It will also remove "duplicate content" between e.g. `/news` and `/news/index.html`. I used absolute paths, as [Google](https://support.google.com/webmasters/answer/139066?hl=en) recommends it. I don't think that the domain name will ever change, so I hardcoded this into the layout. Also fixed mixed tabs/spaces.

See also neovim/bot-ci#6.