Add introduction and improve Admin API spec

[gvdongen]

We will switch the docs to use another openAPI spec renderer. It might be a good idea to go through the spec in the docs and make some improvements. Right now, some pages are empty (introduction) and some are not very well explained. Might be worth it to give this a quick pass and improve it a bit.
Admin API spec here: https://fix-admin-api-ref.documentation-beg.pages.dev/adminapi/admin-api