Add documentation for chemistry related functionality

[TorkelE]

The compound and reaction balancing functionality has been added in: https://github.com/SciML/Catalyst.jl/pull/651 https://github.com/SciML/Catalyst.jl/pull/657 https://github.com/SciML/Catalyst.jl/pull/667 but are not yet documented, this should be added at some point.

[isaacsas]

@LalitChauhan56 are you still planning to do this?

[isaacsas]

Note that a basic tutorial would just introduce what I summarized in the HISTORY.md file.

[isaacsas]

Also, the macro and balance_reaction should get proper doc strings showing their usage and discussing what they return.

[LalitChauhan56]

Yes, I have a basic draft ready. I'll put up a PR by EOD.

[TorkelE]

now works on master