fix(ci): use modflow6 distribution scripts

[wpbonelli]

Fixes the nightly build following changes to distribution scripts in the modflow6 repo:

• Update CI workflows to reflect removal of build_nightly.py, renaming of make_release.py -> update_version.py, and merger of other functionality into build_docs.py and build_dist.py, both of which have a --development flag toggling a minimal nightly development build rather than a full distribution.

• Remove nightly-build.yml workflow since it creates no artifacts and plays no role in the nightly distribution (it just tested the nightly_build.py script, which no longer exists in the modflow6 repository).

• Remove Intel license info from workflow files, these are reproduced in the modflowpy/install-intelfortran-action repo.

• Add code.json to the distribution

Here are a sample run and release

Plus some updates to README.md:

• add latest tag badge
• add table of contents
• fix description (compiled with Intel not GNU Fortran)