Build the sphinx documentation on windows

[Manangka]

This commit enables building the sphinx documentation on windows.
The make.bat needed to do so was missing and is added in this commit.

To build the documentation from a fresh git clone you need to run these commands:

pixi install
pixi run update-flopy
pixi run get-exes
pixi run setup builddir
pixi run build builddir
pixi run benchmark
pixi run sphinx

I've also cleaned the pixi.toml file. I added the git dependencies on flopy, pymake, modflowapi and modflow-devtools as pypi-dependencies. This change made it possible to remove the install task.

Furthermore I added the rtd environment to the pixi.toml. When building sphinx you now don't have to manually pip install
the requirements.rtd.txt. Instead the task will automatically use the rtd environment

Finally i updated the github pipeline files. The newly generated pixi.lock is using version 5 which needs a newer pixi version. I also removed the pixi run install as this is no longer required

Checklist of items for pull request

• Closed issue #xxxx
• Referenced issue or pull request #xxxx
• Added new test or modified an existing test
• Ran black on new and modified autotests
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated meson files, makefiles, and Visual Studio project files for new source files
• Updated definition files
• Updated develop.tex with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Updated input and output guide
• Removed checklist items not relevant to this pull request

For additional information see instructions for contributing and instructions for developing.

[wpbonelli]

Just a note re: the custom install task. All else looks great to me.