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.
- replace "⋅" not displayed on Windows (for some reason) by "·"
- replace the double space at end of lines by the tag <br>
A translatable string cannot ends by 2 spaces, because they are not really visible
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.