[core-3582] add mid term rate maps docs

[radekzawada]

https://bookingsync.atlassian.net/browse/CORE-3582

[StoneFrog]

@radekzawada What do you think about adding a guide, so it's easier to understand how this should be used/interpreted? I prepared a draft a while ago, feel free to use it if you think it would be helpful. https://github.com/BookingSync/bookingsync-api-docs/commit/9a672b1a5882705dbe39d5fb942cbd061d89d8ff#diff-6e320e1194a0bfcd7e6b711e5b546253R1

[radekzawada]

@StoneFrog thanks! I added the guide

[StoneFrog]

Also is the rentals endpoint docs already updated with mid_term_pricing_active attribute? It's quite important in the context of midterm pricing as tells consumer whether or not they should use midterm pricing for 31+ days long bookings

[radekzawada]

@StoneFrog applied feedback