docs: complete the Treestate RFC documentation

[VolodymyrBg]

Motivation

• Complete the Treestate RFC documentation by addressing all TODO items and filling in missing information
• Improve documentation clarity and organization for developers implementing the treestate feature

Solution

This PR completes the Treestate RFC documentation in book/src/dev/rfcs/drafts/0005-treestate.md by:

• Reorganizing the definitions section into separate sections for Common, Sprout, Sapling, and Orchard protocols
• Adding detailed explanations for Heartwood, Canopy, and NU5 anchor validation rules
• Completing the section on handling state rollbacks for non-finalized chain tips
• Adding a comprehensive reference-level explanation with implementation details
• Adding detailed sections on drawbacks, rationale, alternatives, prior art, and unresolved questions
• Expanding the future possibilities section with concrete ideas

Tests

• Documentation changes only, no code modifications requiring tests

Specifications & References

• Referenced the Zcash Protocol Specification for technical accuracy
• Based on existing implementation details in the codebase

Follow-up Work

• Additional documentation improvements for related components might be needed
• Some implementation details might need to be adjusted as the actual code evolves

PR Checklist

• The PR name is suitable for the release notes.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.
• If the PR shouldn't be in the release notes, it has the
  C-exclude-from-changelog label.