RST Reader: Add multiple header rows

[TuongNM]

This adds support for multiple header rows into tables.

In order to accomplish this, the shared GridTable module was to extended to also be able to handle tables with multiple header rows.
Also adjusted the simpleTableRow function to prevent headless tables from being interpreted as multiple header rows and fixed hlint warnings about redundant $.

Fixes #10338

[jgm]

Thank you! I'm okay with merging this as is, but I'd also be in favor of just changing tableWith so it takes a parser that yields a list of header rows ([[Blocks]] rather than [Blocks]). After all, the Table type accepts this, and most of the formats that use tableWith can have multiple header rows. The formats that can accept these should be updated to allow it. @tarleb any thoughts on that?

[TuongNM]

After looking at it, the Markdown reader is currently the only other reader to use these table functions. So if you would like to change tableWith, it should be fairly easy to adjust the uses inside the Markdown reader, provided I can just leave its current header row behavior as is.

[tarleb]

I concur, changing tableWith seems like the best option.

[TuongNM]

Alright, I changed tableWith and made the existing Markdown reader work with it.

Not sure what happened with the CI tests here. Looks like it tried to download a ghc version which somehow failed?

[jgm]

I'll try re-running the actions.