Products overhaul (#77)

* create directory for products, move products.md there, rename to index.md, create files restricted-audience.md and discounts.md, move content from index.md there, write intros

* fix broken links

* fix broken links

* fix more broken links

* fix more broken links

* add .pages; move "Mixed taxation" to taxes.md, edit and expand

* discounts.md: overhaul chapter on early bird tickets

* fix formatting

* overhaul section on combination products, add screenshot

* add explanation of check-in rule logic

* overhaul section on add-ons and bundles

* overhaul section on add-ons and bundles

* start overhauling section on group discounts

* overhaul section on  discounts

* overhaul section on  minimum order amount

* create sessions.md; move "workshops at a conference" there; overhaul intro and option A

* sessions.md: start overhauling option b

* sessions.md: overhault option c and intro

* overhaul restricted-audience.md

* index.md: improve section on mixed taxation

* fix formatting

* add screenshots for sessions.md

* minor corrections

* discounts.md: implement comments

* taxes.md: implement comments

* sessions.md: implement comments

* rewrite section on categories; make it clear that categories are optional except for add-ons and cross-selling

* rewrite section on categories; make it clear that categories are optional except for add-ons and cross-selling

* overhaul ### Time slots , #### Creating slots, and #### Enabling the week calendar view

* overhaul section #### Check-in

* overhaul section ### Season tickets and #### Option A: Memberships and multiple tickets

* overhaul #### Option B: All-access in a single pass

* small corrections

* add and update screenshots; improve instructions on creating time slots within a single day

* add and update screenshots

* add missing word

* Apply suggestions from code review

Co-authored-by: leiascyr <[email protected]>

* Update docs/guides/products/discounts.md

Co-authored-by: leiascyr <[email protected]>

* Update docs/guides/products/discounts.md

Co-authored-by: leiascyr <[email protected]>

* discounts.md: Implement comments

* index.md: implement comments

* Explain difference between cross-selling and add-on products

* Reverse order in last segment

* Move ### Group discounts to the front

* Implement comments

* Implement comments

* Differentiate ### Season tickets into three options

* Improve ### Season tickets

* Add condition to prevent abuse of season pass under option C

* index.md: Remove tax warning

* Do more work on ### Season tickets

* Do more work on ### Season tickets

* Implement comments

* Implement comments

* Implement comments

* Rework 'Mixed taxation' according to comments

* Archive option A, adapt text accordingly

* Explain Mixed taxation (tax added on top of price)

* Implement comment

* Implement comments

* Fix link to tax warning

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Co-authored-by: leiascyr <[email protected]>

Author: mschrumpf