docs(fundamentals): add discovery service documentation

[vahidvdn]

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?

There is nothing about DiscoveryService

What is the new behavior?

Explained DiscoveryService with possible examples and use cases

Does this PR introduce a breaking change?

• Yes
• No

Other information

[vahidvdn]

@kamilmysliwiec Do you have any idea why the ci is failing?

[micalevisk]

do you mind on addressing the issue #2876 here as well?

I guess we don't need to have an example using SetMetadata directly, in favor of DiscoveryService.createDecorator one as it is too low level.