Set up GitHub Pages to use Workflows

[ajkolenc]

Currently the GitHub Page for this repo uses a build branch to host the static HTML output of Flet. This can be improved to automate the build process without requiring manually updating build files on this branch.

To build the Flet app, run this command in the root of the repository:
flet build web ./source/package/pathways_app/ --base-url /PathwaysGenerator/

This creates a directory in source/package/pathways_app called build that contains the HTML file and other assets. *

Documentation can be served in a subdirectory of the output to allow URL access (e.g. https://deltares-research.github.io/PathwaysGenerator/documentation)

• NOTE: this build command will not produce working output until the pathways_app module references the built Wheel file of the adaptation_pathways module, since Flet requires any referenced code to either be in a Wheel file or subdirectory.

[kordejong]

Fixed by #61