Add bzlmod example to setup in docs (#1446)

This adds the bzlmod example from the release docs here:
https://github.com/bazel-contrib/rules_foreign_cc/releases

As a complete rookie, Google took me to this docs page, thinking the
project didn't "work" with bzlmod and I got turned around until I
noticed the release docs have instructions for how to set up ForeignCc
when using bzlmod. Small fix that maybe helps another beginner :)

Author: jakewins

docs/src/index.md

@@ -14,6 +14,16 @@ rules.
 
 ## Setup
 
+### Using bzlmod
+
+Add the following content to your MODULE.bazel:
+
+```python
+bazel_dep(name = "rules_foreign_cc", version = "{version}")
+```
+
+### Using WORKSPACE
+
 To use the ForeignCc build rules, add the following content to your WORKSPACE file:
 
 ```python