docs: add VIM integration instructions for leaf

[111110100]

• This PR adds integration instructions for using leaf with VIM, including keybindings.

Vim Integration

Add the following to your ~/.vimrc to preview the current Markdown file in a vertical split:

" Preview the current Markdown file in a vertical split using leaf
nnoremap <Leader>md :vertical botright terminal leaf -w %<CR>

Once added, use \md to open a live preview. To switch focus back to the Markdown buffer, press Ctrl+w,h.

[RivoLink]

Thank you so much.