Draft: PoC - Markdown grid table syntax based on the Pandoc syntax

What does this MR do and why?

This is a proof of concept of implementing grid tables, as outlined in #460864, authored by @reinaortega

It basically implements the grid_tables extension in Pandoc. If you have Pandoc installed, you can try it with pandoc --from markdown+grid_tables

This is a port of a Python script into a Ruby filter which can be used in GitLab's Banzai pipeline. While in general it implements the grid table syntax, it is specifically tailored to @reinaortega's needs. For example the addition of the markdown="1" attribute is needed so that can share the markdown source with another markdown processor, which uses that attribute to know if each cell should be processed as markdown.

• example grids
• their online grid table converter. Note that it can only handle one table at a time
• example of a failing document because it's larger than our 15MB limit.