Standardize the order of the rules in the handbook

[dingo-d]

Idea that Juliette sugessted in the PR was to standardize the order of certain parts of a rule in the handbook:

1. CS rules
2. Possible code samples
3. Usage in Core (if needed)
4. PHP feature info (if needed)

Also, the color of the boxes would be:

• [info] (blue) - for information about PHP features and how to use them.
• [alert] (yellow) - for information on whether or not something can be used in WP Core.
• [warning] (red) - for anything really problematic (like using the wp namespace prefix).