Rename toctree.yml -> _toctree.yml
Author: mishig25
Pull request: yes

# What does this PR do?
As suggested by @Pierrci 
> it was easier to distinguish "special" files in the build output before IMO
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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


## Who can review?

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