docs: admin preview and draft preview (#10875)

Thoroughly documents the `admin.preview` feature. Previously, this
information was briefly mentioned in two distinct places, within the
collections config and again within the globals config. This led to
discrepancies over time and was inadequate at describing this feature,
such as having a lack of concrete code examples especially as it relates
to _draft preview_. There has also been confusion between this and Live
Preview.

Now, there is a dedicated page at `/admin/preview` which centralizes
this information into a single document. It also specifically documents
how to achieve _draft preview_ and includes code snippets. This way, we
no longer have to rely solely on the [Draft Preview
Example](https://github.com/payloadcms/payload/tree/main/examples/draft-preview)
for this.

Related: #10798