Display message when page/translation doesn't exist and redirect them to tldr repo for them to add it

[kbdharun]

When a page/translation doesn't exist no action takes place currently, maybe a message can be displayed informing the user the page/translation for the English page doesn't exist and tldr repo can be linked for contributing it.

In our Node client we specify this when a page doesn't exist:

> tldr github
`github` documentation is not available.
If you want to contribute it, feel free to send a pull request to: https://github.com/tldr-pages/tldr