doc: correct unicode-input table generation (#51328)
A couple small fixes and cosmetic improvements to the Unicode table in
the docs:
* Most importantly, makes sure that the resulting object from the
at-eval block is `Markdown.MD`, which which will be necessary for
Documenter 1.0 (#47105). It also fixes the `Markdown.Table` structure --
each of the cells should be an array, not just a string, so it adds one
more layer of nesting.
* Cosmetically, center-aligns the characters, and wraps the latex
commands in `<code>`
