Update files for coturn (#3953)

* Update docs/configuring-playbook-turn.md: add a section for description about installing

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

* Update docs/configuring-playbook-turn.md and a related file

- Edit the introducion based on docs/configuring-playbook-client-element-web.md
- Adopt the commont format by creating the section "Adjusting the playbook configuration"
- Add the section "Extending the configuration"
- Move the section "Disabling Coturn" to the bottom

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

* Fix capitalization: Coturn → coturn

See: https://github.com/coturn/coturn. Note that "coturn" is not capitalized even on the start of a sentence, except some rare cases like on the releases page: https://github.com/coturn/coturn/releases

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

---------

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>
Co-authored-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

roles/custom/matrix-coturn/tasks/validate_config.yml

@@ -1,6 +1,6 @@
 ---
 
-- name: (Deprecation) Catch and report renamed Coturn settings
+- name: (Deprecation) Catch and report renamed coturn settings
   ansible.builtin.fail:
     msg: >-
       Your configuration contains a variable, which now has a different name.
@@ -15,7 +15,7 @@
       Invalid authentication method specified in `matrix_coturn_authentication_method`
   when: "matrix_coturn_authentication_method not in ['auth-secret', 'lt-cred-mech']"
 
-- name: Fail if required Coturn settings not defined
+- name: Fail if required coturn settings not defined
   ansible.builtin.fail:
     msg: >-
       You need to define a required configuration setting (`{{ item.name }}`).