notmuch: move the vim plugin to another output

I wondered why my neovim was slow. Turned out notmuch.vim loading took >
500ms to load (ruby and all). And I dont even use it !
I suspect the plugin could be improved to lazyload more stuff but I
think it's ok to have the vim plugin installer be a user decision as well.
I moved it to a new "vim" output : you can install the plugin via
"notmuch.vim"

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -27,6 +27,11 @@
   [v1.7.0](https://github.com/jtroo/kanata/releases/tag/v1.7.0)
   for more information.
 
+- the notmuch vim plugin now lives in a separate output of the `notmuch`
+  package. Installing `notmuch` will not bring the notmuch vim package anymore,
+  add `vimPlugins.notmuch-vim` to your (Neo)vim configuration if you want the
+  vim plugin.
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
 ## Other Notable Changes {#sec-release-25.05-notable-changes}