Update custom UDF location guidance for WinUI3 unpackaged (#3845)

* Update custom UDF location guidance for WinUI3 unpackaged

* Remove app name in example

* from virtual headings to h4s

* restore virtual headings in tabs

* flag "leave this or clean it up" for clarif

* restore some virtual headings

* no need for local toc

* doc how it works now

* doc the present, 2

* specify ADOs, del "review" cmts

* resolve todo re: del folder

* h4s to h2s; leverage "In this article"

* inc answers to PR Comments

* distributing as unpackaged

---------

Co-authored-by: Michael Hoffman <45407486+mikehoffms@users.noreply.github.com>

Author: elibroftw