Installation instructions on docs/ are outdated

[jeongyoonlee]

Describe the bug

docs/installation.rst does not reflect the updates made in #637 and contains outdated instructions.

To Reproduce

Check docs/installation.rst as well as installation instructions on readthedocs (https://causalml.readthedocs.io/en/latest/installation.html#install-from-source).

Expected behavior

docs/installation.rst provides the up-to-date installation instructions.

Screenshots

Environment (please complete the following information):

• OS: [e.g. macOS, Windows, Ubuntu]
• Python Version: [e.g. 3.6, 3.7]
• Versions of Major Dependencies (pandas, scikit-learn, cython): [e.g. pandas==0.25, scikit-learn==0.22, cython==0.28]

Additional context