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`.

after/ftplugin/c.vim

@@ -15,5 +15,3 @@ nnoremap K :YcmCompleter GetDoc<CR>
 let b:surround_{char2nr("t")} = "TODO(\r):"
 " "ys{motion}/" turns "text" into "/*text*/"
 let b:surround_{char2nr("/")} = "/*\r*/"
-" Map K to get YouCompleteMe documentation.
-nnoremap K :YcmCompleter GetDoc<CR>

after/ftplugin/cpp.vim

@@ -1,5 +1,3 @@
 " Add Doxygen documentation generation plugin.
 packadd DoxygenToolkit.vim
 nnoremap <leader>d :Dox<CR>
-" Map K to get YouCompleteMe documentation
-nnoremap K :YcmCompleter GetDoc<CR>

after/ftplugin/python.vim

@@ -25,6 +25,3 @@ setlocal encoding=utf-8
 " Set up file format
 setlocal fileformat=unix
 let g:python_highlight_all=1
-
-" Mappings
-nnoremap K :YcmCompleter GetDoc<CR>

after/ftplugin/vim.vim

@@ -1,9 +1,6 @@
 " Add omnifunc completion package.
 packadd vimomni
 
-" Mapping for Vim help of the word under cursor.
-nnoremap K :help <C-r><C-w><CR>
-
 " Set custom fold expression
 setlocal foldmethod=expr
 setlocal foldexpr=VimFold(v:lnum)