Add OpenMercantil developer API resource

[PabloCirre]

I would like to suggest OpenMercantil as a developer resource.

Suggested insertion, alphabetized under O:

{
    name: 'OpenMercantil',
    description:
        'Independent Spanish public-data API for company search, BORME registry-event timelines, downloads and source methodology.',
    categories: ['API Building', 'Database', 'Documentation'],
    url: 'https://openmercantil.es/api/documentacion',
    keywords: [
        'open data',
        'public data',
        'Spain',
        'Spanish companies',
        'company registry',
        'BORME',
        'REST API',
        'OpenAPI',
    ],
},

OpenMercantil provides API documentation and an OpenAPI file for Spanish company public data and BORME event timelines.

Disclosure: related-party suggestion from the OpenMercantil project. If a PR is preferred, I can prepare one following the repository format.

Important caveat: OpenMercantil is independent and informational; it is not the BOE, BORME or Registro Mercantil.