docs(samples): add GraphQL Federation samples

[Flusinerd]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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
[ ] CI related changes
[ X ] Other... Please describe: Sample apps for GraphQL Federation (#4686)

What is the current behavior?

No samples available for graphql-federation

Issue Number: #4686

What is the new behavior?

Added three sample apps for code-first and schema-first approach each according to the federation docs

Does this PR introduce a breaking change?

[ ] Yes
[ X ] No

Other information

N/A

[coveralls]

Pull Request Test Coverage Report for Build bcd68310-1d95-420e-a50e-8e88fc03a4de

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.468%

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Totals
Change from base Build 19371806-7a0c-4700-bb78-0ee08bdf48a9:	0.0%
Covered Lines:	5499
Relevant Lines:	5821

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💛 - Coveralls

[Flusinerd]

Looks like the samples CI pipeline fails, because the samples I added are three sub folders inside the sample/*/ directory.

Any idea on how to restructure the samples?

[kamilmysliwiec]

Thanks for your contribution! Maybe we can just update the samples script instead of restructuring the example?

[Flusinerd]

That could be a option as well 👍🏻

[Flusinerd]

Okay I modified the gulp file for the samples: a39b2c2
Now it checks if the directory contains a package.json otherwise goes down one directory

[Flusinerd]

Finally got it working, sorry about the mess in the commit headers :)

[kamilmysliwiec]

Thanks @Flusinerd! I'll review this PR as soon as I can

[kamilmysliwiec]

Thank you!