docu(@nestjs/swagger) Describe @ApiSchema decorator

[radacovsky]

Types and Parameters page has been enriched with information about @ApiSchema decorator. The page has been renamed to Model schema in the menu.

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• [x] The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

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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?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[radacovsky]

Done, please recheck

[drFabio]

What is blocking this PR? Just because I have interest on the 983 implementation

[radacovsky]

@johannesschobel I updated documentation according to your comments.