[Docs] Add python processing locally

[sunethwarna]

This PR adds capability to add python snippet output to the docs automatically. Following is the documentation on how to use it.

1. Add ## POST_PROCESS_PAGES_PYTHON_OUTPUT_GENERATION anywhere in the python snippet. It should not have any leading or trailing spaces/tabs.
2. Run the local build in your computer [Make sure to initialize kratos environment or any other library environments in the terminal which is being used by the snippet]. This will run the snippet and capture the output. It will also remove the ## POST_PROCESS_PAGES_PYTHON_OUTPUT_GENERATION tag line from the python snippet.
3. Now you will see some block after the python snippet which contains the python output.

This generation is only done if process_pages.py is passed with -t local flag.

** This will not change any existing python snippets. If the automated output is required, the above tag needs to be placed within the python snippet. **

🆕 Changelog

• Adds capability to automatically add python snippet output.

[philbucher]

Some comments :)

[philbucher]

Suggested change

from KratosMultiphysics.testing.utilities import GetPython3Command

unused

[sunethwarna]

It is used within the method following this line, I did the import their so, specially in the CI, you don't need to have Kratos environment to depoly the pages.

[sunethwarna]

Fixed the pushing of cpp outputs by adding "-u" arg. @philbucher could you check it ? Thanks :)