Rewrite docs pages that describe Sensu's reliance on etcd

[hillaryfraley]

Description

As of Sensu 7.0.0, etcd will no longer depend on etcd. Docs must be updated as a result.

Category

This is an issue with:

• [ ] errata (error in doc content)
• [ ] new doc (requires a completely new doc)
• [x] update (add missing or refresh existing content)
• [x] improvement (improve docs content or format)

Affected pages

Every page that mentions etcd, which will be a substantial number of pages

Additional context

https://github.com/sensu/sensu-go/pull/4915