[HealthSamurai/sansara#8017] Update docs after /fhir/FHIRSchema removal

[ryukzak]

What

Actualizes the docs for the removal of the POST/PUT /fhir/FHIRSchema REST endpoints (HealthSamurai/sansara#8028, refs HealthSamurai/sansara#8017), which also disables FHIRSchema entries in transaction bundles and init bundles.

• custom-resources-using-fhir-schema.md, extensions-using-fhirschema.md — added a "Removed in the July 2026 release" warning pointing to the StructureDefinition-based tutorials; existing instructions are scoped to Aidbox versions 2606 and earlier (kept for stable/LTS users).
• custom-resources/README.md — the FHIRSchema option in the mechanisms list is marked as 2606-and-earlier.
• run-aidbox-+-fhir-portal-locally.md — this guide runs aidboxone:edge with pull_policy: always, so its init bundles would break outright:
  • client-created-by / client-status extension entries converted from FHIRSchema to StructureDefinition in both init bundles, with context added (required by FHIR invariant sdf-5; the FHIRSchema format never required it).
  • AwsAccount / GcpServiceAccount definitions removed instead of converted — they are built-in Aidbox system resources now, and a duplicate definition breaks instance creation with Found more than one schema for type 'AwsAccount'.

Verification

All converted definitions were tested against a live local Aidbox: extension SDs with context are accepted, a Client with both meta-extensions validates, and AwsAccount/GcpServiceAccount instances work against the built-in types with no custom definition.