Netplan: Fix docs directory structure to reflect Diátaxis

[rkratky]

Context

The Netplan docs don't have a Diátaxis-based sub-directory structure, which makes it unnecessarily difficult to find one's way around the docs source code. Also, the Netplan docs contribution guide talks about this (non-existent) directory structure, which is incorrect and confusing.

Task

• Create a Diátaxis based sub-directory structure under the doc/ directory in the Netplan repo
• Move existing docs articles to their respective sub-directories (based on the article type)
• Ensure all respective TOCs and links have been updated to reflect the new locations

Reference

• Diataxis: How-to guides
• ReadTheDocs: Netplan documentation
• GitHub: netplan

[davidekete]

Hi @rkratky, I'd like to work on this