Regression in rendering of reStructuredText tables

[sudonatalie]

A number of tables in our RST documentation are no longer rendering correctly in the GitHub preview: https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst

These tables have have had no changes in several years, but the preview regression began sometime in the last few months. The issue appears to be in particular with tables that have merged cells.

Examples:

• https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#implicit-location-number-assignment
• https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#casts
• https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#hull-entry-point-attributes