fix: Refresh invoice management How-To guide

We now show how to change the email address of an account in the
guide regarding account data, so we remove the corresponding
section from the invoice management guide. While at it, we adopt
the one sentence per line policy, use Font Awesome codes for
referring to UI icons, and introduce minor changes in the prose.

docs/howto/account-billing/manage-invoices.md

@@ -1,66 +1,33 @@
 # Managing invoices
 
-You may quickly review your invoices regarding the monthly usage of
-{{brand}} resources, selectively pay outstanding invoices, save any
-number in PDF format, and change the email address for receiving such
-invoices.
+You may quickly review your invoices regarding the monthly usage of {{brand}} resources, selectively pay outstanding invoices, and save any number of them in PDF format.
 
-First, fire up your favorite web browser, navigate to the
-[{{gui}}](https://{{gui_domain}}) start page, and log into your
-{{brand}} account.
+Using your favorite web browser, navigate to the [{{gui}}](https://{{gui_domain}}) start page and log into your {{brand}} account.
 
 ## Listing invoices
 
-Make sure the vertical pane on the left-hand side of the dashboard is
-expanded. Please look at the options and click on _Invoices_ (right
-below _Users_). Then, in the main pane of the dashboard, you will see a
-list of paid and outstanding invoices.
+Make sure the vertical pane on the left-hand side of the dashboard is expanded.
+Click on _Invoices_, right below _Users_.
+In the main pane of the dashboard, you will see a list of paid and outstanding invoices.
 
 ![All invoices](assets/mgmnt-inv/shot-01.png)
 
-Any paid invoice has a green circle-with-tick icon at the left of its
-row. Also, look at the _Outstanding Amount_ column, where there is a
-value of `0.00` for each paid invoice.
+Any paid invoice has a green :material-check-circle: icon at the left of its row.
+At the _Outstanding Amount_ column, for each paid invoice there is a value of `0.00`.
 
 ![Paid invoices](assets/mgmnt-inv/shot-02.png)
 
 ## Displaying invoice details
 
-Notice the three-dot orange icon at the right of any invoice row. By
-clicking on it, you get a pop-up with specific options regarding the
-corresponding invoice. For example, you can view the invoice's PDF or
-download it onto your computer.
+Notice the orange :material-dots-horizontal-circle: icon at the right of any invoice row.
+By clicking on it, you get a pop-up with specific options regarding the corresponding invoice.
+For example, you can view the invoice or download it onto your computer in PDF format.
 
 ![Options regarding selected invoice](assets/mgmnt-inv/shot-03.png)
 
 ## Paying outstanding invoices
 
-There is also the _Pay this invoice_ option. Choose it for an
-outstanding invoice and you will get a pop-up with a detailed rundown
-of the dues, together with supported payment methods.
+There is also the _Pay this invoice_ option.
+Choose it for an outstanding invoice, and you will get a pop-up with a detailed rundown of the dues, together with available payment methods.
 
 ![Payment options](assets/mgmnt-inv/shot-04.png)
-
-## Changing your invoice email address
-
-Finally, you may change the email address for receiving invoices. At
-the right-hand side of the dashboard, click the user icon at the top
-of the page. A pane titled _Account settings_ will slide over.
-
-There are three tabs in that pane:
-
-* _Customer Info_,
-* _Contact_,
-* _Settings_.
-
-Click the middle one, _Contact_. If there is only one email address, it
-will have all available roles activated --- including the _Billing_
-role.
-
-![Contact info](assets/mgmnt-inv/shot-05.png)
-
-Add a new email address to receive future invoices in a different email.
-Then, make sure the _Billing_ role is deactivated for the old email
-address and activated for the new one.
-
-![New email address for getting billing info](assets/mgmnt-inv/shot-06.png)