docs: Improve code block styles

[adombeck]

• Use ini style code blocks for systemd unit files

  The plain code blocks had some strange highlighting, like highlighting the "bin" in ExecStart=/usr/bin/snap run authd-google -vv.

• Use single backticks for inline code

  Markdown uses single backticks for inline code and three backticks for code blocks.

• Add code block styles to some blocks