Add do#show_documentation() using coc.nvim

Change the `K` normal mode overrides to use the example given in the
coc.nvim README file which uses Vim's builtin help when in `vim` or
`help` files, or checks if coc.nvim is ready and invokes
`CocActionAsync('doHover')`, otherwise fallback to using `keywordprg`.

plugin/mappings.vim

@@ -1,9 +1,10 @@
 " coc.nvim
-nmap <leader>fi <Plug>(coc-fix-current)
-nmap <leader>gd <Plug>(coc-definition)
-nmap <leader>gt <Plug>(coc-type-definition)
-nmap <leader>sd <Plug>(coc-diagnostic-info)
-nmap <leader>gr <Plug>(coc-references)
+nnoremap <silent> <leader>fi <Plug>(coc-fix-current)
+nnoremap <silent> <leader>gd <Plug>(coc-definition)
+nnoremap <silent> <leader>gt <Plug>(coc-type-definition)
+nnoremap <silent> <leader>sd <Plug>(coc-diagnostic-info)
+nnoremap <silent> <leader>gr <Plug>(coc-references)
+nnoremap <silent> K :call do#show_documentation()<CR>
 
 if has('nvim')
   " Make nvim :terminal more like vim :terminal
@@ -110,8 +111,6 @@ nnoremap <C-s> i<C-g>u<C-X>s
 
 " Disable 'Q' from opening Ex mode
 nnoremap Q <nop>
-" Disable 'K' from loading man pages
-noremap K <nop>
 
 " Split line at the cursor
 nnoremap [j i<CR><Esc>