docs: added missing usage of cmp

mappings/custom-mappings.mdx

@@ -86,3 +86,24 @@ Now, you can use `<leader>h1` to go to tab 1, instead of `alt` + `1`.
 
 To create custom mappings using `lua` you can use the `hooks.after_setup` entry in `better-vim.lua` file, following this [tutorial](https://neovim.io/doc/user/lua-guide.html#lua-guide-mappings)
 from the Neovim official documentation.
+
+### Custom mappings for autocomplete (cmp) 
+
+
+To create custom mappings for autocomplete (`cmp`) using `lua`, you can use the `cmp` entry in your configuration. Check out the example below:
+```lua better-vim.lua
+M.cmp = {
+  mappings = {
+    scroll_docs_down = "<C-d>",
+    scroll_docs_up = "<C-u>",
+    next_item = "<C-n>",
+    prev_item = "<C-p>",
+    show_list = "<C-Space>",
+    confirm = "<CR>",
+  },
+  before_default_sources = {},
+  after_default_sources = {},
+}
+```
+
+