Clean up documentation of the base MTC example

[jpn--]

There is documentation for the inputs of the demo "MTC" model here:

https://activitysim.github.io/activitysim/examples.html#inputs

But it isn't formatted well and it is hard to read and follow. These docs could benefit from an effort to tidy them up and include links to MTC's documentation for the original TM1[.5] that has data dictionaries, including for skims as added in #541, additional explanations for model structure, etc.

[stefancoe]

@jpn-- @bricegnichols and I put together the attached skim definition spreadsheet and we are hoping it can be the starting point for a table that gets added to the documentation. This is based on our understanding of things, so it needs to be validated. One thing that we are not totally sure of is units for each skim. @FlaviaTsang could you verify/fill in the Units of Measurement column? Thanks! MTC_skim_dictionary.xlsx