[docs][`pouchdb-find`] Obsolete reference to selector documentation (Mango queries)

The our link I'm discussing is `[CouchDB _find documentation](https://docs.couchdb.org/en/stable/api/database/find.html)`:
https://github.com/pouchdb/pouchdb/blob/f2c665a2a885437b9cea80dda62c02a93a137c1e/docs/_guides/mango-queries.md?plain=1#L348

### Problem:
Couchdb changed the address of the documentation page in version 3.5. It was
```
https://docs.couchdb.org/en/stable/api/database/find.html
```
now in 3.5 it's
```
https://docs.couchdb.org/en/stable/ddocs/mango.html
```

### Proposed solution:
So I suggest that we should either change the link to version 3.4
```
https://docs.couchdb.org/en/3.4.3.post1/api/database/find.html
```
or refer to the new address in 3.5
```
https://docs.couchdb.org/en/stable/ddocs/mango.html
```
P.S. I haven't watched for differences between versions (if any), so I don't know which link to recommend keeping.