This PR's specific commits starts at: cd2c512
The EuiPageTemplate are slightly broken at this point, don't look there yet 🙈

[Breaking] Changed default padding of EuiPage from m to none

• And removed nested page header styles when within EuiPageBody (improved way of doing this coming later)

[NEW] EuiPageSection to replace EuiPageContent & EuiPageContentBody

This component handles all the restrictWidth and border options for page sections to ensure proper extension of the border.

Deprecated EuiPageContent & EuiPageContentBody

[NEW] Added euiPaddingStyles and euiBackgroundColorStyles global styling functions

I've added these new helpers in an attempt to consolidate any component's usage of padding or background colors to use a single global style option. Right now these require passing in the EuiTheme, but once #5754 merges, we'll have an easy way to create a hook version too. (That's also when we'll publicly document it).

Basic usage:

const inlinePadding = euiPaddingStyles(useTheme, 'inline');
const blockPadding = euiPaddingStyles(useTheme, 'block');
const colors = euiBackgroundColorStyles(useTheme);

Docs

• Updated Page section to include page body and sidebar descriptions
• Added a section for new Page sections
• Added a section about complex configurations of page components

Checklist

• Checked in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs and playground toggles
• Added documentation
• Checked Code Sandbox works for any docs examples
• Added or updated jest and cypress tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• Updated the Figma library counterpart
• A changelog entry exists and is marked appropriately