doc

- develop.txt is for design/guidelines; architecture/concepts should
  live elsewhere (currently src/nvim/README.md)
- move dev-jargon to intro.txt
- replace https://neovim.io/community (deprecated) with
  https://neovim.io/#chat
- <Cmd> avoids CmdlineEnter/Leave
  https://github.com/vim/vim/issues/2889

test/functional/api/vim_spec.lua

@@ -1237,7 +1237,7 @@ describe('API', function()
 
   describe('nvim_list_uis', function()
     it('returns empty if --headless', function()
-      -- --embed implies --headless.
+      -- Test runner defaults to --headless.
       eq({}, nvim("list_uis"))
     end)
     it('returns attached UIs', function()