chore(docs): separate docs dependencies

[artemrys]

Issue number: N/A

Summary

Changes

Moving docs-related dependencies out of pyproject.toml into a separate requirements-docs.txt file so we could update docs dependencies separately (otherwise we depend on Python 3.7).

It would allow us to have auto theme for docs which was introduced in mkdocs-material v9.5.0 (https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#automatic-light-dark-mode) which is not available if Python 3.7 is used.

User experience

N/A

Checklist

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• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

[vtsvetkov-splunk]

would it not mess up venv for docs and venv for the rest of the repo? I consider moving everything related to docs to docs/, including (or only?) mkdocs.yml

Then we can create .python-version file that can be picked up by pyenv and github action and have the docs/venv for better isolation

What do you think?