Windows: product names, the editors, WSL; the Linux page's image step

Assembled from development commit c3c40f6c8fca. Since 830b193: on Windows the
products carry Windows names (ktext.exe, klammertext.dll, document.dll,
reflect.dll) and the console's output code page is UTF-8 for a run; the
editor tooling works on Windows (VS Code, Sublime Text, Vim, with the
interpreter's name chosen by platform) and Notepad++ joins it
(doc/edit/notepadpp: a User Defined Language and two Run-menu commands);
a sixth install page, doc/install/windows_wsl_install.md, for Linux
inside WSL; the Windows page opens with the three routes and gains its
editor section, the PowerShell quoting and Path notes; the Linux page
installs OpenImageIO from the python3-openimageio package and verifies
with kdiag --machine; the README lists all six install pages. Linux and
macOS behaviour is unchanged.

doc/edit/vim/autoload/klammertext.vim

@@ -11,7 +11,8 @@
 " Configuration:
 "   g:klammertext_edit_py   full path to klammertext_edit.py (overrides the
 "                           search described below)
-"   g:klammertext_python    the Python interpreter (default "python3")
+"   g:klammertext_python    the Python interpreter (default "python3";
+"                           on Windows "python", the name python.org installs)
 
 " --- locating the shared core ---------------------------------------------
 " Search order: an explicit g:klammertext_edit_py; a vendored copy at this
@@ -37,7 +38,8 @@ function! s:EditScript() abort
 endfunction
 
 function! s:Python() abort
-  return exists('g:klammertext_python') ? g:klammertext_python : 'python3'
+  return exists('g:klammertext_python') ? g:klammertext_python
+        \ : (has('win32') ? 'python' : 'python3')
 endfunction
 
 " Run the core CLI over the whole buffer.  ARGS is the argument string