Fix terminology: distinguish ELM (ontology) from EDC (serialization)

[larinyo]

Summary

The documentation throughout the repository incorrectly used ELM (European Learning Model) when referring to the credential format itself. ELM is the ontology (the data model), while EDC (European Digital Credentials) is the serialization format based on that ontology.

This PR corrects the terminology across all 24 documentation files:

• Root README.md: Updated the supported standards table, credential format labels (ELMv3 → EDC), and the standards & compliance section to properly distinguish between the ELM ontology and the EDC credential format.
• */elm/README.md (8 files): Updated titles and descriptions from "European Learning Model (ELM) credential format" to "European Digital Credentials (EDC) format, a serialization of the ELM ontology". All content item descriptions updated accordingly (ELM-compliant → EDC-compliant, etc.).
• */README.md credential-level READMEs (8 files): Updated the elm/ directory description from "European Learning Model (ELM) schemas" to "European Digital Credentials (EDC) schemas".
• */input-fields/README.md (7 files): Updated credential map descriptions from "the ELM [credential] fields" to "the EDC [credential] fields".

What is NOT changed

• File and directory names (elm/, elm-*-schema.json, etc.) remain unchanged to avoid breaking existing references and tooling.
• JSON schema files that reference "ELM" in the context of the ontology (e.g., "compatible with the full ELM 3.2 schema") are left as-is, since those correctly refer to the ontology.

Test plan

• Verify all internal markdown links still resolve correctly
• Confirm no JSON schema or example files were modified
• Review that ELM is only used when referring to the ontology, and EDC when referring to the credential format