Update website to reflect Mx F hero retirement

[jflowers]

Summary

Mx F (Flow Facilitator) is being retired as a hero persona (see unbound-force/unbound-force#566). The website needs to be updated to reflect this change.

Work Required

• Remove Mx F from the hero roster on the homepage and any hero/agent documentation pages
• Update any references to "five heroes" to reflect the new count
• Review blog posts for stale Mx F references

Context

Mx F's metrics capabilities belong to the infrastructure layer (FullSend), its coaching capabilities were never implemented, and its pipeline integration was never wired. See unbound-force/unbound-force#566 for full rationale.