Feat/customizer panels

[abaicus]

Summary

• chore: rename Layout customizer panel to Global
• fix: dashboard customizer section style
• fix: dashboard responsive & skeleton loader
• chore: hide customizer promo section when starter content is displayed
• feat: reorganize customizer panels and sections
• chore: update starter content H2 typography
• chore: compact docs and promo sections in customizer

Will affect visual aspect of the product

YES (only in customizer)

Screenshots

Dashboard customizer section:

Customizer panels:

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #2915.

[pirate-bot]

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