Add docs requirements to docs group in pyproject.toml

[Zethson]

• Add docs requirements to a new docs group in pyproject.toml
• Remove any requirements.txt in the docs folder if there is any
• Adapt the RTD configuration file to use the docs group
• Adapt the Github workflows accordingly

[Zethson]

Example: theislab/ehrapy#437

[Zethson]

This will only work with the cli-python template because it's the only one using poetry :/