Add ad hoc benchmark results

[dnadales]

This commit adds a new file which will contain the UTxO HD feature documentation. The document is bootstrapped with the latest ad-hoc benchmarks for replaying and syncing up to slot 75M.

The outdated content from docs/utxo-hd.md was moved to docs/utxo-hd-old-integration-plan.md

Checklist

• Branch
  • Commit sequence broadly makes sense
  • Commits have useful messages
  • The documentation has been properly updated
  • New tests are added if needed and existing tests are updated
  • If this branch changes Consensus and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If this branch changes Network and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If serialization changes, user-facing consequences (e.g. replay from genesis) are confirmed to be intentional.
• Pull Request
  • Self-reviewed the diff
  • Useful pull request description at least containing the following information:
    • What does this PR change?
    • Why these changes were needed?
    • How does this affect downstream repositories and/or end-users?
    • Which ticket does this PR close (if any)? If it does, is it linked?
  • Reviewer requested