docs(@nestjs/swagger): add description on how to expose swagger json file

[3ximus]

Docs weren't complete on exposing json spec with using only @nestjs/swagger

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] 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
• [x] Docs
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

This documentation change adds an option that wasn't documented before for exposing json spec file with swagger. The documentation made it seem like it's only possible with other extensions however there is an option with just @nestjs/swagger to expose the document.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

I've been looking on how to do this for a while and couldn't find a working solution anywhere so I think this would be useful.

Cheers