If a page doesn't exist, returns a suggestion to request the command on tldr-pages

**Is your feature request related to a problem? Please describe.**
tldr-pages is a community driven project, so it is important to encourage another users to request or create nonexistent command pages.

**Describe the solution you'd like**
Add a link to open an issue (`<https://github.com/tldr-pages/tldr/issues/new>`) if the command page doesn't exist.