FEP-888d: Using w3id.org/fep as a namespace for extension terms and for FEP documents

[silverpill]

@trwnh At fep issue tracker there’s a new comment about a possible bug in examples: #83 - [TRACKING] FEP-888d: Using https://w3id.org/fep as a base for FEP-specific namespaces - fediverse/fep - Codeberg.org