Allow CSS to target layout of a single page (for docs, blog, pages)

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Description

I went trough documentation and i'm unable to find an option to disable stickiness of navbar for specific page/s

this refers to class `.navbar--fixed-top`

as an example of page that i'd like to disable stickiness is a typescript-eslint [playground](https://typescript-eslint.io/play/#ts=4.7.1-rc&sourceType=module&code=JYMwBAFALgTgrgUzAMmWAjGA1BglGAbzAF8BYAKCA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6ZfKsugIwHs+SAIZN6AD2LQUySnyaoMAbXDYciaND7RIAGhWqsq7JCHx4fAO4BlTswDm6MADNTyRHqPHT5iwDlYALY86o4u8G4enlDelv5mQjxIAPI8AFaI3KGuiPrYAL65ALqROAQk5FQ09MiMLKxMfLRO5kL49rSSfAGUbgpQ6praJVBlpMgU1HQMzGwNtN3IsG4AJh2aCyiOkANaurl4RGMTVdN1c7BMS4irAG5C0H3bGrvDB+XjlVM1M-WNPEJuWj4RocKhMBwYJ6DSAqAp5IA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA)



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i'm not sure if this is a documentation request or feature request

### Self-service

- [ ] I'd be willing to address this documentation request myself.