Add initial documentation on input data

Add initial documentation on input-data file format.
Complete input and output sections

Complete input and output sections of 'Solve' section of documentation.

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Add documentation on span arguments

Add documentation on span arguments (required `--span` argument as
well as optional `--past` argument).
Correct 'important' message

Correct 'important' message to have two dashes in `--past` argument
reference.

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Complete optional-arguments documentation

Complete optional-arguments documentation in 'solve' section. Also add
note about necessary input-data span.

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