docs: replace interfaces with type aliases on configuration page

[micalevisk]

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?

Examples on https://docs.nestjs.com/techniques/configuration are taking in count @nestjs/config@1.1.0

What is the new behavior?

After this change on @nestjs/config: nestjs/config@b85f734 we can no longer use TypeScript interfaces on ConfigService type arg due to TSC constraint errors (see microsoft/TypeScript#42825 (comment))

Does this PR introduce a breaking change?

• Yes
• No

Other information

I chose to update only the interface that will be passed to ConfigService, not the ones used by ConfigService#get, let me know if this is fine

[kamilmysliwiec]

Actually, that's a regression that our e2e should have caught. I've just reverted that commit & will think how to address this issue.