Clean documentation of Units

The documentation about units should be improved in some aspects:

• Implement exponents as exponents;
• Improve italicizing of constants;
• Correct some typos.

Optionally also:

• Add multiplying factors for the derived/allowed units: gram, percent, ppm, minute, litre, nautical mile...;
• Prettify tables.

assigned to @DHutzschenreuter

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changed title from Clean documentation to Clean documentation of Units

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Thanks Giacomo for the feedback. It is well noted and I will check to make an update of this documentation soon.

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marked the checklist item Exponents are in-line; they should implemented as superscripts (line 32 ff.). as completed

marked the checklist item Exponents are in-line; they should implemented as superscripts (line 32 ff.). as incomplete

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marked the checklist item Check footnotes ppm percent bit and byte as completed

marked the checklist item Check footnote binary prefixes as completed

marked the checklist item Exponents are in-line; they should implemented as superscripts (line 32 ff.). as completed

marked the checklist item Some quotes are redundant or wrongly curved (lines 12, 23, 24, 56, 70, 99, 112) . as completed

marked the checklist item Allowed non-SI units miss the multiplying factors (lines 62-70). as completed

marked the checklist item Allowed non-SI units miss the multiplying factors (lines 62-70). as incomplete

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marked the checklist item (not implemented) Allowed non-SI units miss the multiplying factors (lines 62-70). as completed

marked the checklist item According to the SI brochure, the first fundamental unit is named "metre", the British/French way. as completed