docs: fix headings in 'Overlay step' explanation
Metadata
Current evaluation
No evaluation has been recorded for this issue yet.
Issue body
- [X] Have you followed the guidelines for contributing?
- [X] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)?
~~Have you successfully run `make lint && make test`?~~
~~Have you added an entry to the changelog (`docs/reference/changelog.rst`)~~
---
These top-level headings were appearing the explanation category ToC
<img width="234" height="485" alt="image" src="https://github.com/user-attachments/assets/cde10bdc-d6ec-4b1e-9595-52a208d7d5e9" />
Evaluation history
No evaluation history available.