Add core reactive operator reference table

[banchan86]

Closes #117.

I went with the format in my last comment on that issue as I think it achieves a good balance between the following aims for both new and experienced users:

• Quickly finding or viewing available reactive operators
• Gaining enough information to determine their usefulness
• Avoiding an overwhelming amount of detail or clutter

This PR covers all reactive operators in the Bonsai.Reactive namespace, except for the scheduling operators (which seem quite niche, though I may be wrong) and the Sink operator (which was difficult to classify but is included under the broader operator categories).

This is intended as a quick reference akin to the shortcut table, in the future I plan to expand the reactive documentation with #127.

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