Accessibility improvements

[Duncaen]

• Skip to content add skip to content link #61
• unbind mdbooks arrow key bindings? I think this could interference with users who use the keyboard/arrow keys to navigate web pages.
• sitemap? not supported by mdbook.

[pltrz]

When browsing using qutebrowser, which uses insert-mode for typing, typing into the search bar causes it to exit insert-mode after each character typed

[Duncaen]

The search functionality comes from mdbook without any modifications, I don't really want to divert from it.
https://github.com/rust-lang-nursery/mdBook/blob/master/src/theme/searcher/searcher.js#L270-L277
https://github.com/rust-lang-nursery/mdBook/blob/master/src/theme/searcher/searcher.js#L333-L334
They do unfocus the input and refocus it after every key event for some reason.