Introduce code sample tests, improve user agents, update dependencies

This MR introduces rudimentary tests of code snippets via GitLab CI:

• I moved full scripts from Markdown files to dedicated Python files in the tutorials/code directory.
• These scripts are imported into page content during build.
• They are also tested using a dedicated Blubber variant: test-code-samples, using one of the earliest supported versions of Python (3.9.2). This test runs the scripts (via test.sh) one by one after installing dependencies from requirements.txt - tests pass when no errors occur during script execution.

I have also made changes to user agents in full scripts and code snippets to better accommodate the tests and match the access policy. Finally, I updated some JS dependencies.