Add information about payments

Currently there isn't much information about registration fees and payments. The only information available is a section under "Configuring the Registration Process", under "Conferences", which is not the ideal place for it, thus a new page should be created.
Moreover, the new registration fee editing functionality added in indico/indico#6138 should be documented in this new page.

Start with the existing payment information in the "Configuring the Registration Process" section under "Conferences", then review indico/indico#6138 for the registration fee editing functionality. Create a dedicated payments page containing the existing registration fee and payment information plus the new functionality, and update the documentation structure so the content is easy to find.

Written by the indexing model from the issue text.