Add 1.6.0 release notes

[drudge]

Release notes for 1.6.0, written for someone upgrading from 1.5.1. They roll up the five betas into the final state and leave out anything that changed between betas, such as the Check a Domain facts that came and went, and fixes to panels that are new in 1.6. Developer-only changes (Air's live reload, AGENTS.md) are left out too.

Two commits:

1. Add 1.6.0 release notes: the CHANGELOG.md section, dated today. If the release lands on another day, I'll update the date.
2. Point the docs at 1.6.0: docs/index.md, docs/navigation.json, and the Technitium migration guide, which the docs test requires once 1.6.0 is the newest stable section.

Checks

• bash scripts/release-notes.sh v1.6.0
• go tool mage validateReleaseVersion 1.6.0
• go test ./docs/, which also checks that stable notes never mention betas, commit hashes, or pull requests
• Every control the notes name in bold was checked against the console.