Improve documentation on several points

[rintisch]

Feature Request

Is your feature request related to a problem? Please describe. Some features are still not or not clear enough documented;

Describe the solution you'd like Improve documentation of:

• [x] How to use own email template and partials (when #327 is solved)
• [x] Instructions for order plugin → Storage page needs to be set in plugin settings!
• [x] Example how to include the miniCart plugin (see #323)
• [x] taxClassId = -2: Improve explanation
• [x] Flex prices: Explain the different types of plugin.tx_cart.shipping.countries.<country>.options.<n>.extra
• [x] Document option to add different taxClasses for different countries and fallback (see ServiceInterfaceTest.php)
• [x] Summary of when to use EXT:cart and when not to use it.
• [x] Remove section about variants (will be moved to EXT:cart_products.