docs(examples): clarify MongoDB prerequisites in Mongo-backed examples (#15860)

## Summary
Clarifies local setup instructions so MongoDB-backed examples explicitly
state that a running MongoDB instance is required before starting the
app.

Closes #15857

## Related
payloadcms/payload#15857

## Changes
- Adds explicit MongoDB prerequisite guidance to
`examples/custom-components` Quick Start.
- Applies the same guidance to other MongoDB-backed examples with
missing setup notes (`auth`, `custom-server`, `draft-preview`, `email`,
`form-builder`, `live-preview`, `localization`, `tailwind-shadcn-ui`,
`whitelabel`).
- Aligns example setup messaging more closely with template README
conventions for local database prerequisites.

## Testing
- Not run (documentation-only changes).

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Co-authored-by: German Jablonski <GermanJablo-figma@users.noreply.github.com>