Migrate dev and docs from optional dependencies to PEP 735 dependency groups

[lochhh]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
Closes #955

What does this PR do?

• moves docs and dev dependencies from "optional dependencies" (extras) to "dependency groups"
• updates pip install instructions to use --group
• adds the all group that installs both docs and dev
• temporarily uses build_sphinx_docs@build-docs-group-dep (from Use docs group dependency from pyproject.toml in build_docs action actions#158)

References

The NIU build docs action needs to be updated as well (neuroinformatics-unit/actions#158) as it currently uses the old [docs] syntax to install docs dependencies.

How has this PR been tested?

• build docs@main fails
• build docs@build-docs-group-dep passes

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4309245) to head (65a124c).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #956   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           39        39           
  Lines         2691      2691           
=========================================
  Hits          2691      2691           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[niksirbi]

Thanks @lochhh! This was overdue.

At first I was wondering why the test action didn't break, but then I realised because the env for that is installed by tox which is configured in pyproject.toml, so all good.

Should we open the same PR in the python-cookiecutter and at least in our other behavioural repos? (so we don't run the risk of confusing the dev installation syntax across projects).