Fixes #3996 Update docs with new features

[tznind]

@tig is there a way to build and test the docs locally? I see theres lots of ways to link to other pages but seems hard to ensure there are no broken links.

Fixes

• Fixes Track v2 'headline features' #3996

Proposed Changes/Todos

• I noticed the 'ConfigurationManager' link is broken on current live https://gui-cs.github.io/Terminal.GuiV2Docs/docs/newinv2.html

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

Yes. There's a readme in the docfx folder.

[tig]

Also, ShadowStyle.