docs(swagger): add api extension to swagger docs

[tomchinery]

This commit adds documentation for Open API Extensions via the @ApiExtension decorator as per
nestjs/swagger#536.

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
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: Adds docs for: nestjs/swagger#536

What is the new behavior?

Adds docs for @ApiExtension decorator.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[johnbiundo]

LGTM!