Add 1.6.0-beta.5 release notes

[drudge]

Release notes for 1.6.0-beta.5.

What's in it

• Insights drawers name their range in the header (Say which range an Insights drawer covers #270).
• Amazon's ntp-g7g.amazon.com and AWS's time.aws.com count as time sync (Count Amazon's time servers as time sync #272).
• The Check a Domain panel drops its allow and block list facts, and its custom blocked domains chip goes to the Blocked tab (Clear up what a domain check says about lists #269).
• Result notices are green, amber, or red by what they mean (Show a successful SSO provider test in green #271, Color the remaining result notices by what they mean #273).

Checks

• bash scripts/release-notes.sh v1.6.0-beta.5
• go tool mage validateReleaseVersion 1.6.0-beta.5
• go test ./docs/