Todo: Docu Improvements

A reminder to myself: As discussed per Skype on Nov 12, 2020, and later

Available Products:

- [x] disambiguate "rebate product" vs "discount product" in fixedRebate voucher section;
- [x] clarify that a "rebate product" is only there to have a way to add images and texts to specific voucher types. Other fields, like price, are ignored! Do not add them to a category!
- [ ] "Preset" Coupons - better description, add szenario

Global:

- [x] remove all mentions of "... available since...".

various:

- [x] Fix dashes here: https://aimeos.org/docs/2020.x/frontend/html/email-components/
- [ ] Update info on Klarna Sofort (that it is currently not working)
- [ ] Add something about "Groups" (Aimeos menu "Groups") to manual docu

Configuration variablesnot in docu:

- [ ] taxflag (comes from docu, old docu had it; https://aimeos.org/docs/Configuration/Core/mshop/price/taxflag)
- [x] TYPO3 has more configs available that does not show up in docu. Point to php file

Voucher:

- [ ] Provide more explanation for each field. Leave "Reference" field empty.