docs: add OEP-69 REST API Conventions consolidating FC-0118 ADRs

Consolidate the FC-0118 REST API standardization ADRs
(openedx-platform docs/decisions/0025-0037, plus the 0017
authentication pointer ADR) into a single Best Practice OEP, as
requested during community review of the ADRs.

The OEP captures 13 conventions covering DRF serializers, ViewSets,
endpoint consolidation, JWT authentication, permission classes,
idempotent GETs, pagination, filtering/sorting, response shaping,
error responses, API documentation, versioning, and the canonical
MFE configuration endpoint.

Refs: openedx/openedx-platform#38137

Author: Abdul-Muqadim-Arbisoft