Add OpenAPI / Swagger as a skill, closes #18

[anttiviljami]

This PR adds OpenAPI specification as a skill to the Databases & Servers section under REST API.

OpenAPI

Learn about the OpenAPI specification for describing RESTful APIs

• [ ] OpenAPI specification
• [ ] Tutorial of OpenAPI Specification
• [ ] Improving the developer experience with OpenAPI Specification
• [ ] SwaggerHub - Public list of available OpenAPI definitions
• [ ] openapi.tools - List of available OpenAPI tools

[anttiviljami]

Related issue: #18

[anttiviljami]

Fixed the border colour in fced17e to be the same as other icons. Not sure why GraphQL has a different border?