Add more links to sidebar, including CV/HPT.

[rcurtin]

These changes add a Basics section to the sidebar of the documentation, and also provide direct links to the existing CV/HPT documentation.

I think that with these changes, the documentation is ready to deploy, and we can have the Documentation link on the mlpack website link directly to the index.md page.

Compiled example:

click

I thought about having the Basics section expanded by default in the sidebar, but I'd love to hear others' opinions on that too---and any other details.