chore: added OAS3 in the stadanrds and used href pointing to it

[peppelinux]

This PR resolves #847

[Copilot]

Pull Request Overview

This PR updates documentation to standardize terminology by replacing "OpenAPI 3.0" references with a linked "OAS3" abbreviation. The changes introduce a new standard entry in the standards table and update all relevant documentation files (both Italian and English versions) to use the new linked reference format.

Key Changes:

• Added OAS3 as a standard in the standards table with its specification URL
• Replaced multiple instances of "OpenAPI 3.0" text with linked "OAS3" references across documentation files

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/common/standards.rst	Adds OAS3 entry to the National Data Interoperability Platform Specifications table
docs/common/common_definitions.rst	Adds OAS3 hyperlink reference definition pointing to OpenAPI 3.0.3 specification
docs/it/registry.rst	Updates Italian registry documentation to use OAS3 reference instead of "OpenAPI 3.0"
docs/it/entity-onboarding.rst	Updates Italian onboarding documentation to use OAS3 reference in multiple locations
docs/en/registry.rst	Updates English registry documentation to use OAS3 reference instead of "OpenAPI 3.0"
docs/en/entity-onboarding.rst	Updates English onboarding documentation to use OAS3 reference in multiple locations

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Corrected spelling of 'stadanrds' to 'standards' in the PR title.