[git] review section post mkdocs migration

[practicalli-johnny]

Ensure the Git section is using the Material for MkDocs syntax, rather than the older GitBook syntax

• images to recreate (light/dark variants)
• admonitions
• in-page tabs
• link address relative to book root
• link names with semantic icons

Related #331

[practicalli-johnny]

Update up to but not including the Magit Client section
https://practical.li/spacemacs/source-control/magit/

[practicalli-johnny]

Light screenshots only on:

• https://practical.li/spacemacs/source-control/git-configuration/#diff-3-support
• https://practical.li/spacemacs/source-control/forge-configuration/#encrypted-forge-connection-in-authinfogpg

Missing screenshots

• https://practical.li/spacemacs/source-control/magit/create-local-repository/