svMultiPhysics: Add documentation describing the valid range of node and element IDs in text files

[ktbolt]

Problem

The node and element IDs used in text files are assumed to be 0-based but this is not documented.

Solution

Add documentation to the Data File Formats section describing how to specify node and element IDs within text files.

Probably also need to say something about this for VTK files too.

Additional context

No response

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