feat: adds rover docs open and rover docs list

[EverlastingBugstopper]

fixes #308 with one modification

$ rover docs list
You can open any of these documentation pages by running rover docs open <slug>.

+--------------+---------------------------------+
| Slug         | Description                     |
+--------------+---------------------------------+
| api-keys     | Understanding Apollo's API Keys |
+--------------+---------------------------------+
| docs         | Rover's Documentation Homepage  |
+--------------+---------------------------------+
| start        | Getting Started with Rover      |
+--------------+---------------------------------+
| contributing | Contributing to Rover           |
+--------------+---------------------------------+

TODO: the webbrowser crate we use relies on some environment variables across operating systems. We should likely document this.

[lrlna]

Niiiicee!