Windows is installed through WSL; the native Windows build is withdrawn (from dev a9756dac74c6)

Klammertext on Windows is the Linux build inside WSL 2, or the prebuilt
containers through Docker Desktop. The native build from source (MSYS2) is
no longer offered:

- doc/install/windows_source_install.md and the bundled shell under win/
  are removed.
- doc/install/windows_wsl_install.md is the Windows page. It says there is
  no native build, and its editor section says what holds for each editor:
  Emacs, Vim and Visual Studio Code (through its WSL extension) as on
  Linux; Sublime Text and Notepad++ on the Windows side, with their
  self-contained packages, on files opened through \\wsl.localhost.
- The README lists six install guides; the Notepad++ and VS Code READMEs
  no longer refer to the MSYS2 shell.

No change to the language, the Standard Klammer Set or the commands.

doc/edit/notepadpp/README.md

@@ -24,8 +24,11 @@ current file, and this package uses exactly those two things.
 
 - Notepad++ 7.9 or later (the `userDefineLangs` folder; any recent
   version qualifies).
-- For the Run commands: `python` on the `Path`, which the Klammertext
-  installation already needs.
+- For the Run commands: `python` on the `Path` -- Python for Windows
+  from python.org, with "Add python.exe to PATH" selected. The commands
+  are Python and do not need Klammertext itself, which on Windows lives
+  in WSL (the install page `windows_wsl_install.md`); a file there is opened
+  through `\\wsl.localhost\Ubuntu\...`.
 - Files saved as UTF-8 (Notepad++'s default for new files; **Encoding ￫
   Convert to UTF-8** for an older one). Line endings do not matter, and
   a file in another encoding is refused by `ktext` with the remedy named.