Add docs artifact directory to .gitignore

Adds the docs directory that is generated for documentation output to the .gitignore file to
prevent contributors from accidentally commiting it, and to keep `git status` clean.

Signed-off-by: Winford <winford@object.stream>

Author: UncleGrumpy

.gitignore

@@ -11,3 +11,4 @@
 **/erl_crash.dump
 !priv/.gitignore
 !examples/myapp/.gitignore
+docs/**