Bug fixes

Base theme

• Issue #3 "Unknown "dd" function"
• Removed outdated documentation.
• Added new variable main_layout_attributes contains empty object Attribute() for main layout modifiers on demand.

Starter theme

Fixed code style and comments.
Fixed hook_help example
Added perform.api.php file with description about hooks and alters.

Fixed issues:
#2904359 by Chi: Wrong permission condition in perfmon_page()
#3008848 by yuraosn: Notice: Undefined variable: severity in function perfmon_requirements()

Changes since 8.x-3.11:

Changes since 8.x-3.1: