[Docs][Feature] Add the ability to search through the documentation

The documentation is still a bit messy and it's hard to find where concepts are documented. For instance, I was trying to find information about slide fragments, but nothing showed up in the API Reference.

### Description

Add the ability to search through the documentation.

### Proposal

This could be done via a search bar at the top of the page. This could rely on Algolia, since Spectacle is an opensource project.

### Links / References

- Algolia:
- https://www.algolia.com/pricing/ -> It's free while the userbase is small
- https://www.algolia.com/for-open-source/ -> It should be free as long as Spectacle is Open-Source