The next PR after #2350 and #2343.

This PR contains the following changes:

1. Update SDK from v1.57 to v1.58a.
2. Improved the Shell Makefile (the part related to the Steamworks extension).
   • Moved building the fake API process inside a separate Makefile specific to the fake API.
3. Cleaned some comments including flushing the Steamworks Makefile.
4. Fixed the README.md file.
5. Improved documentation.
6. Created a separate Makefile for the fake Steamworks API.
7. Added v1.58a official headers.
8. Fixed .gitignore file.