feat(dev): Add docs about OCP\IPhoneNumberUtil

Signed-off-by: Joas Schilling <coding@schilljs.com>

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_28.rst

@@ -58,6 +58,8 @@ Added APIs
 * ``\OCP\Preview\BeforePreviewFetchedEvent::getHeight``
 * ``\OCP\Preview\BeforePreviewFetchedEvent::getMode``
 * ``\OCP\Preview\BeforePreviewFetchedEvent::getWidth``
+* ``\OCP\IPhoneNumberUtil::convertToStandardFormat`` to convert input into an E164 formatted phone number. See :ref:`phonenumberutil` for an example.
+* ``\OCP\IPhoneNumberUtil::getCountryCodeForRegion`` to get the E164 country code for a given region. See :ref:`phonenumberutil` for an example.
 
 Changed APIs
 ^^^^^^^^^^^^

developer_manual/digging_deeper/index.rst

@@ -40,3 +40,4 @@ Digging deeper
    user_migration
    profiler
    deadlock
+   phonenumberutil

developer_manual/digging_deeper/phonenumberutil.rst

@@ -0,0 +1,65 @@
+.. _phonenumberutil:
+
+=================
+Phone number util
+=================
+
+``OCP\IPhoneNumberUtil`` is a wrapper around the third-party library `libphonenumber <https://github.com/giggsey/libphonenumber-for-php>`_.
+It is simplified to the most common use cases to allow replacing the library in the future without having to break the
+public API and functionality.
+
+Convert input into standard format
+----------------------------------
+
+To convert a phone number to E164 format, call ``convertToStandardFormat()`` with a known region. The input can also
+contain formatting characters, such as spaces, slashes and dashes:
+
+.. code-block:: php
+
+  $input = '044 / 668-1800';
+  $util = \OCP\Server::get(\OCP\IPhoneNumberUtil::class);
+  var_dump($util->convertToStandardFormat($input, 'CH'));
+  // Will output:
+  // string(12) "+41446681800"
+
+When no region is given and the phone number can not be mapped to a single region, converting will fail:
+
+.. code-block:: php
+
+  $input = '044 668 1800';
+  $util = \OCP\Server::get(\OCP\IPhoneNumberUtil::class);
+  var_dump($util->convertToStandardFormat($input, null));
+  // Will output:
+  // NULL
+
+The phone number can also be provided in an international format containing the region code. In this case, the default region is ignored:
+
+.. code-block:: php
+
+  $input = '+41 44 668 1800';
+  $util = \OCP\Server::get(\OCP\IPhoneNumberUtil::class);
+  var_dump($util->convertToStandardFormat($input, null));
+  var_dump($util->convertToStandardFormat($input, 'DE'));
+  // Both will output:
+  // string(12) "+41446681800"
+
+Get the country code for a region
+---------------------------------
+
+To check if a provided region is valid (2-letter code of ``ISO 3166-1``) and has a country code use ``getCountryCodeForRegion()``:
+
+.. code-block:: php
+
+  $util = \OCP\Server::get(\OCP\IPhoneNumberUtil::class);
+  var_dump($util->getCountryCodeForRegion('DE'));
+  // Will output:
+  // int(49)
+
+Again ``null`` is used to indicate invalid input:
+
+.. code-block:: php
+
+  $util = \OCP\Server::get(\OCP\IPhoneNumberUtil::class);
+  var_dump($util->getCountryCodeForRegion('Germany'));
+  // Will output:
+  // NULL