document stimulus tables input format

This format will document how we represent stimulation epochs in files prior to packaging into NWB. The document should describe:

- location, format, and name of file(s)
- all columns and acceptable column values

Place this documentation in aind-file-standards/file_formats/stimulus_tables.md

Start with the template: https://github.com/AllenNeuralDynamics/aind-file-standards/blob/main/core/template.md