[Docs] Optimize mercure docs

[fashxp]

Studio API version

Steps to reproduce

• mention in installation guide that before installing studio-backend-bundle, mercure needs to be set up - otherwise you get error messages that mercure parameters are not defined
• volume configuration in ngnix config: - ./studio-demo:/var/www/html:ro doesn't apply to our default setup, shouldn't it be - .:/var/www/html:ro
• https://github.com/pimcore/studio-backend-bundle/blob/1.x/doc/02_Mercure_Setup.md#check-if-mercure-is-running --> it says Unauthorized for me

Actual Behavior

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Expected Behavior

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