Turn on search

[huijing]

Closes #364

This PR turns on search by adding the required configuration options in the docusaurus config file.

🎩 Top-hatting instructions (i.e. human testing that things are not broken)

1. Pull down this branch
2. Navigate to http://localhost:3000/ (or you could probably use the deploy preview since we turned that on ¯\_(ツ)_/¯) and click on the search bar in the top right corner
3. The overlay should pop-up without errors, and if you start typing, there should be results showing up until you reach a string that we do not have indexed

[netlify[bot]]

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