Update UI coding conventions and guidelines

[erri120]

Resolves #737.

• new docs for building UIs
• Updated the FOMOD installer UI as an example on how to apply the new guidelines
• Updated various extension methods to be more strict

[codecov]

Codecov Report

Merging #738 (8b9b5c1) into main (af773e8) will increase coverage by 0.22%.
The diff coverage is 0.48%.

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
+ Coverage   62.43%   62.66%   +0.22%     
==========================================
  Files         586      585       -1     
  Lines       17505    17422      -83     
  Branches     1217     1219       +2     
==========================================
- Hits        10930    10917      -13     
+ Misses       6255     6184      -71     
- Partials      320      321       +1     

Flag	Coverage Δ
Linux	61.94% <0.48%> (+0.23%)	⬆️
Windows	61.85% <0.48%> (+0.21%)	⬆️
clean_environment_tests	62.64% <0.48%> (+0.22%)	⬆️
network_tests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...UI/Options/GuidedInstallerOptionDesignViewModel.cs	0.00% <ø> (ø)
src/NexusMods.App.UI/AViewModel.cs	100.00% <ø> (ø)
src/NexusMods.App.UI/ServiceExtensions.cs	100.00% <ø> (ø)
...ls/Helpers/ViewModelSelector/AViewModelSelector.cs	100.00% <ø> (ø)
...lpers/ViewModelSelector/ViewModelDesignSelector.cs	100.00% <ø> (ø)
...els/Helpers/ViewModelSelector/ViewModelSelector.cs	100.00% <ø> (ø)
...ceSystem/Workspace/WorkspacePlaygroundViewModel.cs	0.00% <ø> (ø)
...OD.UI/Group/GuidedInstallerGroupDesignViewModel.cs	0.00% <0.00%> (ø)
...OMOD.UI/Step/GuidedInstallerStepDesignViewModel.cs	0.00% <0.00%> (ø)
src/NexusMods.App.UI/ReactiveUIExtensions.cs	65.21% <25.00%> (-4.56%)	⬇️
... and 4 more

... and 2 files with indirect coverage changes