Add sandbox for new documentation

[quic-kyunggeu]

Main changes

Created a sandbox workspace for the new doc in Docs/beta.

$ tree Docs/beta
Docs/beta
├── _static
│   └── style.css
├── conf.py
└── index.rst

Notes

• You can build the new doc with make doc_beta
• I copy-pasted conf.py and style.css from the current doc just as a placeholder. Anyone can feel free to completely rewrite any of these
• Currently it's just a hello-world example