[Book] make headlines consistent with our standards
[Reference] consistent headlines
[QuickTour] consistent headlines
minor #3765 [WIP] make headlines consistent with our standards (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[WIP] make headlines consistent with our standards

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

- [x] Book
- [ ] Components
- [ ] Contributing
- [ ] Cookbook
- [x] Quick Tour
- [x] Reference

Commits
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1d2a620 [QuickTour] consistent headlines
d49d12e [Reference] consistent headlines
0702d1f [Book] make headlines consistent with our standards