[Best Practices] update best practices index

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

This way, the table of contents on the index page has only one level
like every other index in the documentation.
Everywhere else, this path is absolute. I'm not sure if it'll matter, but we might as well be absolute here too to avoid issues.