docs(openapi): document OpenAPI 3.2 hierarchical tags

[apt-bh]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines:
  https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

The OpenAPI "Operations" page only documents the string form of @ApiTags and the basic
DocumentBuilder.addTag(name, description?, externalDocs?) signature. There is no mention of the
parent / kind fields introduced in OpenAPI 3.2, nor of the object form of @ApiTags.

Issue Number: N/A

What is the new behavior?

The "Tags" section of content/openapi/operations.md now documents:

• DocumentBuilder.addTag() with the new optional 4th argument for hierarchy options (parent, kind)
• The object form of @ApiTags({ name, parent?, kind? })
• Mixing string and object forms
• A hint clarifying OpenAPI 3.0/3.1 backward compatibility and that hierarchy info is only meaningful at
  the document level

Related feature PR: nestjs/swagger#3725

Does this PR introduce a breaking change?

• Yes
• No

Other information

• Docs updated: content/openapi/operations.md