docs(openapi): prefer lazy over eager creation of the document

[H4ad]

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?

NestJS does not use the new feature of lazy initialization of Swagger, which adds some time to startup.

Associated with nestjs/swagger#2392

What is the new behavior?

Now, by default, we use always the lazy initialization in favor of the initialization speed.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[Tony133]

This linked PR:nestjs/swagger#2392 is associated with this one and has been merged, so I would say merge this as well, right?

[micalevisk]

I guess we can merge this now 😸

[H4ad]

@kamilmysliwiec Can we merge this PR, this change was merged a long time ago.