docs: re-add Validate AsyncAPI documents

[derberg]

resolves #1864

adding https://www.asyncapi.com/docs/guides/validate to v3

the only changes from oryginal document:

1. Sentence One way this can be done is by using Spectral, an API linting tool which has a built-in custom ruleset properties with AsyncAPI rules for the AsyncAPI specification. It also enables you to define company-specific rules that you can use internally. to One way this can be done is by using Spectral open source tool. It enables you to define company-specific rules that you can use internally. because we do not know when spectral will have default AsyncAPI v3 rules
2. In 4 steps tutorial, from code samples I removed part where we instruct how to extend default asyncapi v2 rules. Spectral works well without having default support for asyncapi v3. I tested my new instructions on local and was still able to apply some custom lint rules to my asyncapi document

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[quetzalliwrites]

/rtm