[docs]: align component page header and page css to patternfly.org

[bennypowers]

Compare

https://patternflyelements.org/components/accordion/

https://www.patternfly.org/v4/components/accordion/

• the background colour for the main body of the page should be grey
• the overview text should be within the header

depends on #2444

[bennypowers]

So we need to update

• docs/components/components.md - to include the element.description in the header
• elements/*/docs/*.md - remove instances of renderOverview
• possibly tools/pfe-tools/11ty/DocsPage.ts - may need to add a new shortcode to get the element description
• plus the CSS changes