feat: support where querying by join fields (#12075)

### What?
This PR adds support for `where` querying by the join field (don't
confuse with `where` querying of related docs via `joins.where`)

Previously, this didn't work:
```
const categories = await payload.find({
  collection: 'categories',
  where: { 'relatedPosts.title': { equals: 'my-title' } },
})
```

### Why?
This is crucial for bi-directional relationships, can be used for access
control.

### How?
Implements `where` handling for join fields the same as we do for
relationships. In MongoDB it's not as efficient as it can be, the old PR
that improves it and can be updated later is here
https://github.com/payloadcms/payload/pull/8858

Fixes https://github.com/payloadcms/payload/discussions/9683

docs/fields/join.mdx

@@ -271,21 +271,6 @@ const result = await payload.find({
   and blocks.
 </Banner>
 
-<Banner type="warning">
-  Currently, querying by the Join Field itself is not supported, meaning:
-  ```ts
-  payload.find({
-    collection: 'categories',
-    where: {
-      'relatedPosts.title': { // relatedPosts is a join field
-        equals: "post"
-      }
-    }
-  })
-  ```
-  does not work yet.
-</Banner>
-
 ### Rest API
 
 The REST API supports the same query options as the Local API. You can use the `joins` query parameter to customize the