Don't use substripts in HTML if there is just a number in the
subscript: "R1", not "R<sub>1</sub>", except for characteristic
impedance. And of course still use "Z<sub>in</sub>", not "Zin".
This is also consistent with the images.
These help files are created from the .md source files and contain I18n strings,
but do not exist if kicad is not built from sources.
However they are needed by translators.