Edit docs/configuring-playbook-email2matrix.md: adopt the common instruction

Based on docs/configuring-playbook-dimension.md and docs/configuring-playbook-bot-go-neb.md

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

docs/configuring-playbook-email2matrix.md

@@ -22,11 +22,25 @@ If you really need to run an email server on the Matrix machine for other purpos
 
 For details about using Email2Matrix alongside [Postfix](http://www.postfix.org/), see [here](https://github.com/devture/email2matrix/blob/master/docs/setup_with_postfix.md).
 
-### Creating a user
+### Register a dedicated Matrix user (optional)
 
-Before enabling Email2Matrix, you'd most likely wish to create a dedicated user (or more) that would be sending messages on the Matrix side. Take note of the user's ID as it needs to be specified as `MatrixUserId` on your `inventory/host_vars/matrix.example.com/vars.yml` file later.
+We recommend that you create a dedicated Matrix user for Email2Matrix.
 
-Refer to [Registering users](registering-users.md) for ways to create a user. A regular (non-admin) user works best.
+Generate a strong password for the user. You can create one with a command like `pwgen -s 64 1`.
+
+You can use the playbook to [register a new user](registering-users.md):
+
+```sh
+ansible-playbook -i inventory/hosts setup.yml --extra-vars='username=email2matrix password=PASSWORD_FOR_THE_USER admin=no' --tags=register-user
+```
+
+Take note of the user's ID as it needs to be specified as `MatrixUserId` on your `inventory/host_vars/matrix.example.com/vars.yml` file later.
+
+### Obtain an access token
+
+Email2Matrix requires an access token for the sender user to be able to send messages to the room. Refer to the documentation on [how to obtain an access token](obtaining-access-tokens.md).
+
+⚠️ **Warning**: Access tokens are sensitive information. Do not include them in any bug reports, messages, or logs. Do not share the access token with anyone.
 
 ### Creating a shared room
 
@@ -38,25 +52,20 @@ Inviting additional people to the room is okay too.
 
 Take note of each room's room ID (different clients show the room ID in a different place). You'll need the room ID when [configuring the playbook](#adjusting-the-playbook-configuration) below.
 
-### Obtain an access token for the sender user
-
-In order for the sender user created above to be able to send messages to the room, we'll need to obtain an access token for it. Refer to the documentation on [how to obtain an access token](obtaining-access-tokens.md).
-
-⚠️ **Warning**: Access tokens are sensitive information. Do not include them in any bug reports, messages, or logs. Do not share the access token with anyone.
-
 ## Adjusting the playbook configuration
 
-After doing the preparation steps above, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file (adapt to your needs):
+To enable Email2Matrix, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file. Make sure to replace `ACCESS_TOKEN_FOR_EMAIL2MATRIX1_HERE` and `ACCESS_TOKEN_FOR_EMAIL2MATRIX2_HERE` with the ones created [above](#obtain-an-access-token).
 
 ```yaml
 matrix_email2matrix_enabled: true
 
+# You need at least 1 mailbox.
 matrix_email2matrix_matrix_mappings:
   - MailboxName: "mailbox1"
     MatrixRoomId: "!qporfwt:{{ matrix_domain }}"
     MatrixHomeserverUrl: "{{ matrix_homeserver_url }}"
     MatrixUserId: "@email2matrix1:{{ matrix_domain }}"
-    MatrixAccessToken: "MATRIX_ACCESS_TOKEN_HERE"
+    MatrixAccessToken: "ACCESS_TOKEN_FOR_EMAIL2MATRIX1_HERE"
     IgnoreSubject: false
     IgnoreBody: false
     SkipMarkdown: false
@@ -65,7 +74,7 @@ matrix_email2matrix_matrix_mappings:
     MatrixRoomId: "!aaabaa:{{ matrix_domain }}"
     MatrixHomeserverUrl: "{{ matrix_homeserver_url }}"
     MatrixUserId: "@email2matrix2:{{ matrix_domain }}"
-    MatrixAccessToken: "MATRIX_ACCESS_TOKEN_HERE"
+    MatrixAccessToken: "ACCESS_TOKEN_FOR_EMAIL2MATRIX2_HERE"
     IgnoreSubject: true
     IgnoreBody: false
     SkipMarkdown: true