Docs: Use of `.. code-block` for better rendering

[antoinemeyer5]

In this page https://pressio.github.io/rom-tools-and-workflows/generated/romtools.linalg.linalg.html, we want to display examples of ranks, tensors, and others. Using .. code-block:: text and .. code-block:: python improves the display, enables copy-and-paste, and code recognition.

Before	After

This work can be done here:

• https://pressio.github.io/rom-tools-and-workflows/generated/romtools.linalg.linalg.html#romtools.linalg.linalg.argmax
• https://pressio.github.io/rom-tools-and-workflows/generated/romtools.linalg.linalg.html#romtools.linalg.linalg.mean
• https://pressio.github.io/rom-tools-and-workflows/generated/romtools.linalg.linalg.html#romtools.linalg.linalg.min
• https://pressio.github.io/rom-tools-and-workflows/generated/romtools.linalg.linalg.html#romtools.linalg.linalg.std