Migrate, review and update Kernel Git Guide

[AnneCYH]

Scope:

• Migrate existing Kernel Git Guide wiki to RTD
• Review and update relevant content
• Restructure content according to Diataxis principles

Related pages

• https://canonical-kernel-docs.readthedocs-hosted.com/en/latest/how-to/build-publish/build-kernel/#how-to-build-kernel-obtain-source
• https://canonical-kernel-docs.readthedocs-hosted.com/en/latest/how-to/prepare/obtain-kernel-source-git/