VARIOUS: Fix issues mentioned in the feedback

In this MR I wanted to fix some issues mentioned in the reports.

Therefore I have:

a) Implemented paging for /all-like endpoints

b) Added readOnly attribute for @Transactional where applicable

c) Changed saveAndFlush call for save since we use @Transactional

d) Added OpenAPI .yaml for each microservice, so client can be easily generated

e) Added minutes for the Doctor Available, so minutes in Open Hours can be specified