Recent troubles with Windows users:

• gfortran is not installed
  • Required to compile 6S
  • Difficult to install
• bash is not installed
  • Required for 6S simulation executions
  • Difficult to install via WSL

In an effort to ease these pain points, I've attempted the following:

• There's already a precheck for gfortran in data/cli/sixs.py. Since isofit download sixs already supported a --mingw for Windows which is used for make to compile 6S, mingw also comes with gfortran. As such, I've set the precheck to prompt Windows users to install mingw via the CLI before 6S.
• I broke out the mingw install to data/cli/windows.py to keep Windows utility installs together and separate from 6S. With this, I've added a isofit download windows --bash option to install the portable version of Git Bash.
• The mingw and git-bash installs are tracked within the isofit.ini file and inserted into the $PATH at runtime, so they're always available
• isofit download all will download these Windows utilities before anything else to ensure they're available for 6S
• The Git Bash terminal can be spawned via isofit dev bash, which will allow Windows users to follow the same docs instructions as Mac/Linux, hopefully simplifying a lot of headaches
• data/build_examples.py now generates .ps1 scripts for Windows PowerShell as an additional option for users
• Asked AI to generate a document on executing the examples for the various pathways Windows users could experience ([PowerShell, Git Bash, WSL] + uv). I've touched this up so it's accurate, but always want team sign-off on something like this

Closes #847