Support MathML tags for rendering

[Girgias]

DocBook allows use of MathML which could be useful for nicely rendering some mathematical related equations or code.

Was no point to bother previously as Chrome only supports MathML since January 2023.

[cmb69]

We need to consider other output formats, though. E.g. CHM likely fails to render MathML.

[Girgias]

Could use MathJax for CHM according to the internet.