Add docs about redundancy (#2142)

## Issue Addressed

- Resolves #2140

## Proposed Changes

Adds some documentation on the topic of "redundancy".

## Additional Info

NA

book/src/SUMMARY.md

@@ -33,6 +33,7 @@
 	* [Local Testnets](./local-testnets.md)
     * [Advanced Networking](./advanced_networking.md)
     * [Running a Slasher](./slasher.md)
+    * [Redundancy](./redundancy.md)
 * [Contributing](./contributing.md)
 	* [Development Environment](./setup.md)
 * [FAQs](./faq.md)