Merge 'upstream' example of the classic complete keymaps

- Add a commented out example of the classic complete keymaps.
- Minor improvements of debian install instructions.

lua/kickstart/plugins/cmp.lua

@@ -68,6 +68,12 @@ return {
           --  This will expand snippets if the LSP sent a snippet.
           ['<C-y>'] = cmp.mapping.confirm { select = true },
 
+          -- If you prefer more traditional completion keymaps,
+          -- you can uncomment the following lines
+          --['<CR>'] = cmp.mapping.confirm { select = true },
+          --['<Tab>'] = cmp.mapping.select_next_item(),
+          --['<S-Tab>'] = cmp.mapping.select_prev_item(),
+
           -- Manually trigger a completion from nvim-cmp.
           --  Generally you don't need this, because nvim-cmp will display
           --  completions whenever it has completion options available.