Enhance documentation with dynamic config example

[alexkart]

This pull request improves the documentation in docs/basics.md by adding a helpful note on dynamic configuration syntax. The new section demonstrates how to create configuration options that depend on overridden values.

[antonmedv]

Should we introduce an api to "extract" not overridden values?

[alexkart]

Should we introduce an api to "extract" not overridden values?

Maybe. This could be useful for some use cases. However, the note I added is more about the opposite cases—when you do need to use an overridden config value to create a new config option based on it. If you try something like this: set('uses_original_dir_name', '/path/to/' . get('dir_name')); outside of the task context, you’ll get the original value instead of the one passed via the -o option. That’s why I thought it was useful to mention this in the docs, since it’s not immediately obvious.