feat: add instructions about tests

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

developer_manual/getting-started/index.rst

@@ -7,4 +7,5 @@ Getting started
 
    branch-policies
    development-environment/index
+   tests
    commits

developer_manual/getting-started/tests.rst

@@ -0,0 +1,84 @@
+Testing
+=======
+
+LibreSign includes multiple test suites (see ``.github/workflows`` for CI jobs).  
+Below are the most relevant for day-to-day development and how to run them locally.
+
+Unit tests
+----------
+
+Run a specific unit test (filter by class, method, or pattern):
+
+.. code-block:: bash
+
+   composer test:unit -- --filter MyClassTest
+
+.. note::
+   The double dash ``--`` is required to pass arguments to the script
+   ``test:unit`` (and not to Composer itself).
+
+Running the entire unit suite locally may take a while. Prefer filtering by the
+tests you added or modified.
+
+Integration tests (Behat)
+-------------------------
+
+Integration tests live under ``tests/integration``. To run them:
+
+1. Install dependencies inside that folder:
+
+   .. code-block:: bash
+
+      cd tests/integration
+      composer install
+
+2. Run a specific scenario (example):
+
+   .. code-block:: bash
+
+      runuser -u www-data -- vendor/bin/behat --xdebug features/account/me.feature:5
+
+   The example above executes the scenario that **starts at line 5** of
+   ``features/account/me.feature``.
+
+Static analysis
+---------------
+
+**PHPCS** (coding style):
+
+.. code-block:: bash
+
+   composer cs:fix
+
+**Psalm** (type analysis):
+
+.. code-block:: bash
+
+   composer psalm
+
+Update Psalm baseline (only when appropriate):
+
+.. code-block:: bash
+
+   composer psalm:update-baseline
+
+JavaScript linters
+------------------
+
+**ESLint** and **Stylelint**:
+
+.. code-block:: bash
+
+   npm run lint:fix
+   npm run stylelint:fix
+
+Tips
+----
+
+- Prefer running commands **inside the Nextcloud container** as the ``www-data`` user
+  when applicable (permissions and paths will match CI). You may also use the
+  ``runuser`` command, as shown in the examples.
+- Keep your local branches rebased and dependencies up to date to reduce noise in
+  lints and type checks.
+- Tests and CI are also part of the documentation, check ``.github/workflows`` and
+  the ``tests`` folder for usage examples.