Outdated instructions for compiling bitcoin core from source in Appendix II

### Summary
The instructions in [Appendix II: Compiling Bitcoin from Source](https://github.com/BlockchainCommons/Learning-Bitcoin-from-the-Command-Line/blob/8ea3dadc7ae5e3b62b7d6e329724c85f19102ae4/A2_0_Compiling_Bitcoin_from_Source.md) are out of date.

They reference scripts and build tools that have been removed in recent Bitcoin Core releases like `autogen.sh`, `./configure`, and `install_db4.sh`

### Proposed Changes
Update Appendix II to show instructions for current Bitcoin Core build process using `CMake` and the `depends` system,
Emphasize which flags to set for a lightweight node like `NO_QT=1 NO_NATPMP=1 NO_UPNP=1 NO_USDT=1`.
Emphasize which flags to pass to `bitcoind` to run a pruned signet node, as recommended by the course.

I would like to update this documentation and submit a PR, let me know if you're open to these changes.

Start with A2_0_Compiling_Bitcoin_from_Source.md and replace the outdated autogen.sh, ./configure, and install_db4.sh references with the current Bitcoin Core CMake and depends workflow. Document the lightweight-node flags NO_QT=1, NO_NATPMP=1, NO_UPNP=1, and NO_USDT=1, then add the bitcoind flags for a pruned signet node as recommended by the course.

Written by the indexing model from the issue text.