Document and export continuous_events and discrete_events

[MasonProtter]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I'd like a blessed API for fetching the callbacks of an ODESystem without reaching into internals, so I figure marking these functions as exported and giving them docstrings would be a good way to go about this. If there's a better, existing way to do that please let me know.

[ChrisRackauckas]

Seems reasonable to me. Add it to the docs?

[ChrisRackauckas]

Add it to the system docs for the docstrings of the available functions?

[MasonProtter]

Ahh, I see, I missed that section.

[MasonProtter]

Should I remove the mention of it from Events.md and just have it in the system docs?

[ChrisRackauckas]

Yes

[MasonProtter]

Done, let me know if that's a sufficient level of detail. I'm not sure how much we should explain the structure of SymbolicContinuousCallback and SymbolicDiscreteCallback. Maybe just say something like "these encode the first element of the input Pair in the .eqs field and the second element of the input Pair in the .affect field"?

[ChrisRackauckas]

Maybe just say something like "these encode the first element of the input Pair in the .eqs field and the second element of the input Pair in the .affect field"?

That would be good to add.

Looks like formatter is blocking tests.

[ChrisRackauckas]

Odd docs build failure, I'm going to assume that was a freak event, but also do a pass over this repo soon to fix a few red checkmarks.