docs: lsp, vim_diff

- quickstart
- mark lsp.txt as `new_layout`
- remove lsp-handler documentation for notifications: they don't have
  handlers because they don't have server responses.

scripts/gen_help_html.lua

@@ -43,6 +43,7 @@ local M = {}
 -- All other files are "legacy" files which require fixed-width layout.
 local new_layout = {
   ['api.txt'] = true,
+  ['lsp.txt'] = true,
   ['channel.txt'] = true,
   ['deprecated.txt'] = true,
   ['develop.txt'] = true,