Update README.md for Stable Haskell fork

- Remove all references to Hadrian build system
- Replace build instructions with make-based commands
- Add prominent notice that this is the Stable Haskell fork
- Update git clone URL to stable-haskell/ghc repository
- Direct all issues to github.com/stable-haskell/ghc/issues
- Simplify build requirements to GHC >= 9.8.4 and cabal-install
- Add GHCup installation instructions
- Merge HACKING.md content into README.md
- Remove references to haskell-code-explorer.mfix.io
- Remove --enable-tarballs-autodownload mention
- Remove multi-core build note
- Remove orphan markdown links
- Remove reference to upstream build system documentation

Author: andreabedini