Docs: Split structure guide into individual files

doc/user/events/guides/earlybird_numbers.rst

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+Use case: Early-bird tiers based on ticket numbers
+--------------------------------------------------
+
+Let's say you run a conference with 400 tickets that has the following pricing scheme:
+
+* First 100 tickets ("super early bird"): € 450
+* Next 100 tickets ("early bird"): € 550
+* Remaining tickets ("regular"): € 650
+
+First of all, create three products:
+
+* "Super early bird ticket"
+* "Early bird ticket"
+* "Regular ticket"
+
+Then, create three quotas:
+
+* "Super early bird" with a **size of 100** and the "Super early bird ticket" product selected. At "Advanced options",
+  select the box "Close this quota permanently once it is sold out".
+
+* "Early bird and lower" with a **size of 200** and both of the "Super early bird ticket" and "Early bird ticket"
+  products selected. At "Advanced options", select the box "Close this quota permanently once it is sold out".
+
+* "All participants" with a **size of 400**, all three products selected and **no additional options**.
+
+Next, modify the product "Regular ticket". In the section "Availability", you should look for the option "Only show
+after sellout of" and select your quota "Early bird and lower". Do the same for the "Early bird ticket" with the quota
+"Super early bird ticket".
+
+This will ensure the following things:
+
+* Each ticket level is only visible after the previous level is sold out.
+
+* As soon as one level is really sold out, it's not coming back, because the quota "closes", i.e. locks in place.
+
+* By creating a total quota of 400 with all tickets included, you can still make sure to sell the maximum number of
+  tickets, even if e.g. early-bird tickets are canceled.
+
+Optionally, if you want to hide the early bird prices once they are sold out, go to "Settings", then "Display" and
+select "Hide all products that are sold out". Of course, it might be a nice idea to keep showing the prices to remind
+people to buy earlier next time ;)
+
+Please note that there might be short time intervals where the prices switch back and forth: When the last early bird
+tickets are in someone's cart (but not yet sold!), the early bird tickets will show as "Reserved" and the regular
+tickets start showing up. However, if the customers holding the reservations do not complete their order,
+the early bird tickets will become available again. This is not avoidable if we want to prevent malicious users
+from blocking all the cheap tickets without an actual sale happening.