chore: use dependency groups (PEP 735)

[mwtoews]

This PR separates a few optional dependencies into dependency groups, as defined by PEP 735.

The optional dependencies (or "extras") are kept, and are intended for user-facing install options:

• optional: this is the primary group, with documentation and support functions
• codegen: includes other dependencies needed for code generation

Dependency groups are intended for internal or developer-facing install options:

• lint, test and dev are re-listed as dependency groups
• doc is renamed as docs group, plural to match the folder name "docs"
• The dev group is similar to as it was as an extra, but doc (now docs) is removed, as not all developers need to create docs (? maybe?); nevertheless it can be installed via --group docs

A few sections of DEVELOPER.md were revised to describe how to install the extras or dependency groups in different Python environments.

The pytest header (in autotest/conftest.py) is modified to only show "optional" packages and not the "test" dependency group packages. If this is important, it could be re-enabled, but it would need to parse the group directly from pyproject.toml, as this information is not part of the project's metadata.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.3%. Comparing base (556c088) to head (8d1cd00).
⚠️ Report is 201 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2809      +/-   ##
===========================================
+ Coverage     55.5%    73.3%   +17.7%     
===========================================
  Files          644      659      +15     
  Lines       124135   132182    +8047     
===========================================
+ Hits         68947    96909   +27962     
+ Misses       55188    35273   -19915     

Files with missing lines	Coverage Δ
flopy/utils/utl_import.py	68.1% <ø> (ø)

... and 585 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[wpbonelli]

thanks for this