Update links to Sensu docs pages in README and REFERENCE

[hillaryfraley]

Description

• In README.md:
  • Updates Sensu docs link for contact routing
  • Changes "affect" to "effect" to fix broken TOC link
• In manifest backend.pp:
  • Updates Sensu docs link for securing Sensu agent TLS
• In manifest agent.pp:
  • Updates Sensu docs link for agent security config flags

Related Issue

None

Motivation and Context

I found these backlinks because I am working on redirects in the Sensu docs.

[ghoneycutt]

Thanks @hillaryfraley

The REFERENCE.md file is auto-generated so to update those links you change the comments at the top of the manifests. See https://github.com/sensu/sensu-puppet/blob/master/manifests/backend.pp#L53

[hillaryfraley]

I see, thank you! Apologies for missing that...I'll fix it now.