Add Erlang module examples to the introductory notebook (#2831)

Co-authored-by: Jonatan Kłosko <jonatanklosko@gmail.com>

lib/livebook/notebook/learn/intro_to_livebook.livemd

@@ -155,8 +155,6 @@ $$
 S(x) = \frac{1}{1 + e^{-x}} = \frac{e^{x}}{e^{x} + 1}
 $$
 
-<!-- livebook:{"force_markdown":true} -->
-
 To write your own, put your math expressions between \$ signs for inline math
 or \$\$ if you want display math. You can double click the formulas above to see
 how they are written.
@@ -190,13 +188,13 @@ you can leverage all of the dependency management and smart cell features
 outlined in the previous sections. In particular, integration between
 Erlang and Elixir will happen as follows:
 
-  * Variables in Elixir are available in Erlang cells in camel-case
-    fashion. `x` in Elixir becomes `X` in Erlang. `foo_bar` becomes
-    `FooBar`;
+* Variables in Elixir are available in Erlang cells in camel-case
+  fashion. `x` in Elixir becomes `X` in Erlang. `foo_bar` becomes
+  `FooBar`;
 
-  * Variables in Erlang are available in Elixir cells in underscored
-    fashion. `X` in Erlang becomes `x` in Elixir. `FooBar` becomes
-    `foo_bar`;
+* Variables in Erlang are available in Elixir cells in underscored
+  fashion. `X` in Erlang becomes `x` in Elixir. `FooBar` becomes
+  `foo_bar`;
 
 For example, to print all of the cats defined at the top of the notebook,
 but in Erlang:
@@ -205,6 +203,33 @@ but in Erlang:
 [io:format("~ts", [Cat]) || Cat <- Cats].
 ```
 
+### Defining modules
+
+You can also define Erlang modules, one per cell, as you would in an `.erl` file.
+
+```erlang
+-module(direction).
+
+-export([north/0, east/0, west/0, south/0]).
+
+north() -> {ok, north}.
+east() -> {ok, east}.
+west() -> {ok, west}.
+south() -> {ok, south}.
+```
+
+Macros, includes, and similar module feature are supported.
+
+Once the module is defined, you can call the functions as usual:
+
+```erlang
+{ok, N} = direction:north(),
+{ok, E} = direction:east(),
+{ok, W} = direction:west(),
+{ok, S} = direction:south(),
+{N, E, W, S}.
+```
+
 We are just beginning the Erlang integration and contributions to
 further enrich it are welcome.