Minor documentation updates

[aspect]

This PR includes the following updates:

• Linux dependency information for dependencies that may not be available on all distributions: build-essential, libssl-dev, pkg-config
• Splits LLVM information for Windows into a separate section, addressing:
  • https://github.com/kaspanet/rusty-kaspa/issues/246
  • https://github.com/kaspanet/rusty-kaspa/issues/248
• Provides a Git for Windows link, addressing:
  • https://github.com/kaspanet/rusty-kaspa/issues/247
• Adds kaspad/README.md that points to the main repository README.md (needed for https://crates.io/crates/kaspad)