Update manual installation prerequisites

[caydyan]

Addresses CounterpartyXCP/counterparty-core#2434.

This updates the manual installation guide to reflect current Ubuntu/macOS setup expectations:

• uses sudo apt update and sudo apt install for Ubuntu package commands
• includes build-essential and clang in the Ubuntu prerequisites
• installs python3-venv and documents creating/activating a virtual environment
• uses python -m pip from the active virtual environment for Maturin and package installs
• keeps the LevelDB apt command under sudo

Verification:

• npm run build succeeded
• git diff --check passed

Note: the build still reports the existing API v1 broken-anchor warnings; this PR does not touch those pages.

BTC address for bounty/reward consideration: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc