Add pixi tasks

[LuukBlom]

Kind of request

Adding new functionality

Enhancement Description

We want to be able to use pixi tasks to handle stuff instead of using gh actions

Task list

I'd like tasks for the following things:

• build api documentation
• build user guide documentation
• build executable / installer
• run the gui (needs refinement! maybe add the gui repo as a git submodule)

Use case

Any user cloning the FloodAdapt repo will only have to use a few pixi commands:

• pixi install
• pixi update
• pixi run [task_name]

Instead of having to run python scripts to install the environment, (which will then not automatically update itself)
And then having to search for the commands/ files that are required to run the tasks

Additional Context

pixi run tests is already implemented

[kathrynroscoe]

Can't do this until #485 is fixed