Add python3-silo test dependency to installation docs

The Silo file format tests require the Python Silo bindings
(python3-silo on Debian/Ubuntu) to read back and compare generated
.silo files against reference files. Added "For running the test suite"
sections for all Linux platforms.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Lutz Gross

doc/sphinx_api/installation.rst

@@ -200,6 +200,12 @@ For symbolic mathematics support, additionally install:
 
    sudo apt-get install python3-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo apt-get install python3-silo
+
 Build esys-escript:
 
 .. code-block:: bash
@@ -239,6 +245,12 @@ For symbolic mathematics support, additionally install:
 
    sudo apt-get install python3-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo apt-get install python3-silo
+
 Build esys-escript:
 
 .. code-block:: bash
@@ -282,6 +294,12 @@ For symbolic mathematics support:
 
    sudo pacman -Sy python-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo pacman -Sy python-silo
+
 Build esys-escript:
 
 .. code-block:: bash
@@ -323,6 +341,12 @@ For symbolic mathematics support:
 
    sudo dnf install python3-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo dnf install python3-silo
+
 Build esys-escript:
 
 .. code-block:: bash
@@ -360,6 +384,12 @@ For symbolic mathematics support:
 
    sudo zypper in python3-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo zypper in python3-silo
+
 Build esys-escript:
 
 .. code-block:: bash
@@ -409,6 +439,12 @@ For symbolic mathematics support:
 
    sudo dnf install python3-sympy
 
+For running the test suite, additionally install:
+
+.. code-block:: bash
+
+   sudo dnf install python3-silo
+
 Build esys-escript:
 
 .. code-block:: bash