docs: add v-app requirement note to v-app-bar

[mmustafasenoglu]

Description

Adds a warning note to the v-app-bar documentation that the component must be used inside a v-app component.

Fixes #22855

Motivation

Users frequently encounter the injection "Symbol(vuetify:layout)" not found error when using v-app-bar without wrapping it in v-app. The current documentation does not mention this requirement, forcing users to search for solutions externally.

Changes

• Added a warning callout in the Usage section explaining that v-app-bar must be inside v-app
• Included the specific error message users will see
• Linked to the v-app component documentation

[mmustafasenoglu]

Thanks for reviewing!

I've added a note in the v-app-bar documentation clarifying that v-app must be the parent component, otherwise you'll get a injection "Symbol(vuetify:layout)" not found error.

Let me know if you need any changes.