update best practices index

This way, the table of contents on the index page has only one level
like every other index in the documentation.
minor #4352 [Best Practices] update best practices index (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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facea46 update best practices index