docs(VAppBar): document that v-app-bar must be inside v-app

[sridhar-3009]

Summary

• Adds a ::: warning callout to the App bars documentation page, placed directly after the Usage example
• Explains that v-app-bar depends on Vuetify's layout injection system and must be a descendant of v-app
• Reproduces the cryptic injection "Symbol(vuetify:layout)" not found error message users encounter so it is easy to search for
• Provides a minimal correct HTML structure (<v-app><v-app-bar>…</v-app-bar><v-main>…</v-main></v-app>) as a copy-paste guide

Closes #22855

Test plan

• Open the docs page /components/app-bars/ locally (pnpm dev in packages/docs) and confirm the warning callout renders correctly after the Usage example
• Verify the fenced code blocks inside the callout display as expected