Update code explanation dataset to support slash command

What does this merge request do and why?

This MR update the code-explanation dataset to support slash command. It adds 2 new fields:

• code_before_snippet: the code before the selected code
• code_after_snippet: the code after the selected code

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.