docs: add security best practices (#714)

* docs: add security best practices

* docs: improve per style guide

* docs: add xref to nemoclaw

Author: miyoungc

docs/index.md

@@ -174,6 +174,16 @@ Policy schema, environment variables, and system architecture.
 {bdg-secondary}`Reference`
 :::
 
+:::{grid-item-card} Security Best Practices
+:link: security/best-practices
+:link-type: doc
+
+Every configurable security control, its default, and the risk of changing it.
+
++++
+{bdg-secondary}`Concept`
+:::
+
 ::::
 
 ---
@@ -238,6 +248,13 @@ reference/policy-schema
 reference/support-matrix
 ```
 
+```{toctree}
+:caption: Security
+:hidden:
+
+security/best-practices
+```
+
 ```{toctree}
 :caption: Resources
 :hidden: