Widget structure (#112)

* Restructure and improve article, particularly multiple events, series dates, and filtering

* Fix ### Using the widget for an event series and subsections

* Lay out structure for ## Applications

* Guide through restricting availability, preselecting vouchers, and offering discounts through the widget from a user perspective under ## Applications

* Finish restructuring for now

* Improve according to Vale; Rework "Show or hide event info" segment

* Implement more Vale improvements

* Attempt to clear up user/customer confusion

* Turn info on `data-fix="true"` into a note. Split up sentences. Make negation bold. Remove passive

* Restructure #### Using tracking with the pretix Widget

* Provide short content summeries at top of long sections; fix outline

* Implement more Vale improvements

* Implement the last batch of Vale improvements

* Restructure and fix section on user data

* Move section on pretix Button

* Rework subsections on metadata attributes/selections of events or dates

* Improve clarity, formatting, and structure

* Implement comments

* Format notes more sensibly

* Move "troubleshooting" before "versioning"

* Move "Offering discounts through the widget" down

* Restructure and fix whole article

Author: mschrumpf