Improve custom field documentation

I've been trying to create my own "Select images from media gallery" custom field which basically just replicates [this document component block extension](https://github.com/beemstream/keystone-document-gallery), but I feel like I'm basically throwing my head against a wall trying to understand what all the different custom field config options actually do. While I do appreciate that everything is typed, reverse engineering from the types alone is very painful.

Here's some things I think would be very helpful if explained in documentation:

1. What is the config object passed into your custom field function? For example:
```ts
export function text({
isIndexed,
...config // <-- this
}
```
2. Explanation of the `graphql` object exported from `@keystone-6/core`
2. Detailed explanation of the `input` and `output` properties required by the `fieldType()` function
2. How does this magical `controller` object work? How does it hook in to everything else? Maybe some simple diagrams?
2. Detailed explanation of all the properties in the following types:
- `FieldControllerConfig`
- `FieldController`

I'm sure there's more that I've missed, but better documentation for custom fields would be a god-send.

No documentation file is named. Start by tracing the custom field function, the graphql export, fieldType() input and output properties, and the FieldControllerConfig and FieldController types. Done means the custom-field configuration and controller behavior are explained with practical examples, including the requested diagrams where useful.

Written by the indexing model from the issue text.