Add a section on using infura as the checkpoint sync provider (#2797)
Add a section on using infura as the checkpoint sync provider (#2797)

## Issue Addressed

N/A

## Proposed Changes

From discord, it seems like users are a bit unclear on how to run checkpoint sync if they don't have an existing synced beacon node. Adds a note on how to use infura for the checkpoint sync feature.




sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse