Add additional Vanguards documentation

Tasks

Add into the Technology/Security section:

• Tor Vanguards Specification - Tor Specifications.
• Announcing the Vanguards Add-On for Onion Services | The Tor Project.
• proposals/333-vanguards-lite.md · main · The Tor Project / Core / Tor Specifications · GitLab.
• From “Onion Not Found” to Guard Discovery.
• Announcing Vanguards Support in Arti | The Tor Project.

Time estimation

• Complexity: negligible (0.1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)