Phase 1 (P1): Step 1 (S1) - Update Threshold Doc Structure (Outline)

[mattjs777]

Update Doc Structure | Tentative Deadline: 2-3 weeks post-approval

• Update Threshold Doc Structure
• Align on Structure & Hierarchy
• Address Gaps/Extra Sections (tickets)

### Tasks
- [ ] https://github.com/threshold-network/threshold/issues/45
- [ ] Revamp: What is the Threshould Network (include tBTC, thUSD, TACo)
- [ ] Create new image showing the Threashold network with the applications under it
- [ ] Developer Section: Combine [Governance & Development Page] and [Overview] to be Developement, Appilcations & Governance]
- [ ] Developer Section: Why thUSD [security/infrastructure]
- [ ] Developer Section: Why TACo [security/infrastructure]
- [ ] Apps Section: tBTC Use Cases