ocis_ldap deployment example no longer works and requires documentation

## Describe the bug
I'd expect the example to show how to connect and use an external LDAP server. However, there seem to be undocumented requirements.
It's unclear whether we can use the entryUUID or objectGUID as `IDP_LDAP_UUID_ATTRIBUTE`. I remember we need to convert the data in order to search by that attribute in OC10, and I don't know if that's supported here. This would mean that we need to setup a different uuid attribute before connecting the LDAP server in oCIS.

In addition, the example uses the "bitnami/openldap" image, which isn't available any longer, so the provided docker example can't be build at the moment.

## Steps to reproduce
1. Run `docker compose up` in the ocis_ldap example
2.
3.

## Expected behavior
oCIS is up and running

## Actual behavior
bitnami image isn't found and docker compose fails.

## Setup
Please describe how you started the server and provide a list of relevant environment variables or configuration files.

```console
OCIS_XXX=somevalue
OCIS_YYY=somevalue
PROXY_XXX=somevalue
```

## Additional context
Add any other context about the problem here.