docs: add custom admin location docs

[zubricks]

Fixes #13707

• created comprehensive custom admin location guide
• updated admin/overview.mdx with cross-references and FAQ
• updated custom-components overview with import map regeneration details

[AlessioGr]

Move => Rename

[AlessioGr]

One missing step: you will have to change the import in src/app/(payload)/layout.tsx

from

import { importMap } from './admin/importMap.js'

to

import { importMap } from './importMap.js'

if you're unlucky and your IDE does not auto-update imports. Maybe just mention that the user needs to make sure old imports referencing the admin folder need to be updated, so they should run build / typecheck and update imports until it passes.

[AlessioGr]

So if you do this:

mv app/(payload) app/admin/content/(payload)

The admin route will be under /admin/content/admin, not /admin/content. Maybe a bit of a strange example, because the API is not part of the payload admin panel, so might not wanna put it under /admin.

Also unlike the first example, this change moves both the payload API and admin panel, so might wanna clarify that in the heading

## Scenario 2: Nested Admin Panel