Add doc page

Signed-off-by: Julian-Samuel Gebühr <julian-samuel@gebuehr.net>

docs/configuring-playbook-matrix-ldap-registration-proxy.md

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+# Setting up matrix-ldap-registration-proxy (optional)
+
+The playbook can install and configure [matrix-ldap-registration-proxy](https://gitlab.com/activism.international/matrix_ldap_registration_proxy) for you.
+
+This proxy handles Matrix registration requests and forwards them to LDAP.
+
+**Please note:** This does support the full Matrix specification for registrations. It only provide a very coarse
+implementation of a basic password registration.
+
+## Quickstart
+
+Add the following configuration to your `inventory/host_vars/matrix.DOMAIN/vars.yml` file:
+
+```yaml
+matrix_ldap_registration_proxy_enabled: true
+```