Add documentation for LaTeX formatted tables with units - #17627

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And of course t.write.help("ascii.latex") does too, though I wonder how many people would know to try that...

I only discovered that while writing this PR and I've been an io.ascii maintainer for close to a decade... but I frequently look at this table and then click on the linked classes there. However, search engines are probably the most common entry point to our docs.