Use docs group dependency from pyproject.toml in build_docs action

[lochhh]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
This PR is needed as part of the work for migrating docs from optional dependencies to PEP 735 dependency groups (in neuroinformatics-unit/movement#956)
Details are described in the issue: neuroinformatics-unit/movement#955

What does this PR do?
Updates the pip install command to use --group

How has this PR been tested?

This is tested in neuroinformatics-unit/movement#956 CI

• build docs@main fails because of this issue.
• build docs@build-docs-group-dep passes

Is this a breaking change?

Docs build in existing repos using the pyproject.toml option will fail (movement example)

Does this PR require an update to the documentation?

Yes

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[niksirbi]

Thanks @lochhh!

I'm approving this PR, but before merging can you double-check which NIU/BrainGlobe projects use use-requirements-txt: false and depend on this action from the main branch? Those will break upon merging this.

The only one that comes to mind is poseinterface, but there may be others (perhaps ethology)?

I think we should open a PR similar to neuroinformatics-unit/movement#956 also on these other projects.