Minor changes in contribution guidelines

[Alixbonard]

From the openMINDS meeting dev discussion of the 20/04/2026:
@lzehl Minor changes in the contribution guidelines page:

• Add a hyperlink to 'compact-1' in the sentence "Instances are written in JSON-LD compact-1 format." linking to compact-1 explanation in 'Getting started/JSON-LD: minimal introduction' page in formatting section.
• Add a hyperlink to 'POSIX standard' in the sentence "End the file with a trailing newline, in accordance with the POSIX standard." in formatting section.
• Add the structure of metadata schema files ("type", "extends", "required", "properties") in content section.
• Remove 'global' in "The identifiers of instances in the openMINDS library (‘@ id’) are defined as IRIs to ensure their global uniqueness [...]" in the Instance Identifiers section.