Update installation doc

[willingc]

Motivation

This PR clarifies a few installation points that we ran into when installing Docusaurus for https://docs.nteract.io

Contributing back to a project that we find very useful and hope it helps future users.

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Yes

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[yangshun]

@willingc I agree with your proposed changes. Thanks for improving this! 😄

[willingc]

Thanks for the fast review @yangshun. 😄

[yangshun]

@willingc Do let us know which of your projects you are using Docusaurus for by adding it to our Users page.