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/README.md

@@ -14,7 +14,8 @@ sync.
 ## Requirements
 
 - Vim 8+ (or Neovim) with `+eval` — any normal Vim; only `vim-tiny` lacks it.
-- `python3` on `PATH` (Klammertext itself already requires Python).
+- `python3` on `PATH` — on Windows `python`, the name python.org installs
+  (Klammertext itself already requires Python).
 - The shared core, found automatically in this order:
   1. `g:klammertext_edit_py`, if you set it (a full path to
      `klammertext_edit.py`);
@@ -81,7 +82,7 @@ in-process: the `=` operator reindents through `'indentexpr'` (`==`, `gg=G`),
 and the matching delimiter is highlighted live as the cursor sits on one —
 the show-paren equivalent, with a mismatched or unbalanced delimiter shown
 in red plus a message.  Without `+python3` the commands above still work;
-they shell out to `python3`.
+they shell out to `python3` (on Windows `python`).
 
 **Comment toggling** — `'commentstring'` is set to `# %s`, so Vim 9.1's
 built-in commenting and Neovim's `gcc`/`gc` (or the commentary plugin)
@@ -92,7 +93,7 @@ toggle `#` line removal.
 | Variable | Meaning (default) |
 |---|---|
 | `g:klammertext_edit_py` | full path to `klammertext_edit.py` (auto-detected) |
-| `g:klammertext_python` | Python interpreter (`python3`) |
+| `g:klammertext_python` | Python interpreter (`python3`; on Windows `python`) |
 | `g:klammertext_no_mappings` | set to define no default mappings |
 
 ## Neovim and the language server