Fix table formatting in OpenEXRFileLayout.rst

Two tables currently implemented as `grid tables` should be converted to `list-table`, which handles left-justification of text:

- https://openexr.com/en/latest/OpenEXRFileLayout.html#high-level-layout
- https://openexr.com/en/latest/OpenEXRFileLayout.html#version-field

This task requires some basic familiarity with [reStructuredText](https://docutils.sourceforge.io/rst.html) and [sphinx](https://www.sphinx-doc.org/en/master) but the concepts are easy to pick up from other examples. See [Building the Website](https://openexr.com/en/latest/install.html#building-the-website) for how to test the formatting.

Open OpenEXRFileLayout.rst and locate the grid tables in the High-Level Layout and Version Field sections. Review nearby list-table examples and the Building the Website instructions, then build the documentation to verify both tables render with left-justified text.

Written by the indexing model from the issue text.