AsyncAPI documentation URLS point to non-existing resources

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Example of non-existing link: https://github.com/asyncapi/spec/blob/master/versions/2.0.0/asyncapi.md#externalDocumentationObject

Solution outline:

All links must use following prefix for: https://www.asyncapi.com/docs/specifications/v2.3.0. Formatting of documentation Markdown strings has been enhanced, all links have been fixed, bugs and mistakes have been eliminated.

Resources:

• https://markdown-it.github.io/
• https://code.visualstudio.com/docs/languages/markdown