add clean conda env install sequence to docs/installation.rst

[ras44]

Describe the bug
Add the below conda installation sequence to docs/installation.rst (previously removed from README.md).

causalml/README.md

Lines 112 to 120 in 028c63c

	### Create a clean conda environment
	```
	conda create -n causalml-py38 -y python=3.8
	conda activate causalml-py38
	conda install -c conda-forge cxx-compiler
	conda install python-graphviz
	conda install -c conda-forge xorg-libxrender
	```

To Reproduce
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Expected behavior
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Screenshots
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Environment (please complete the following information):
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Additional context
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