Add plain styling support to Wizard component

[thatblindgeye]

Context

This is followup work from patternfly#8215.

Changes in patternfly

• Added plain styling (transparent background) for Wizard components

• New CSS variables for plain modifier: --pf-m-plain__header--BackgroundColor, --pf-m-plain__toggle--BackgroundColor, --pf-m-plain__nav--BackgroundColor, --pf-m-plain__outer-wrap--BackgroundColor, --pf-m-plain__footer--BackgroundColor

• Updated border radius token for nav links to use --pf-t--global--border--radius--action--plain--default

• Auto-applies plain styling when glass theme is enabled (unless .pf-m-no-plain is used)

• Added new "Plain" example in documentation

• Added .pf-m-plain and .pf-m-no-plain modifiers

Work Needed in patternfly-react

• Add `isPlain` prop to Wizard component to apply `.pf-m-plain` modifier

• Add `isNoPlain` prop to Wizard component to apply `.pf-m-no-plain` modifier (to prevent auto-plain styling with glass theme)

• Create a new "Plain" example/demo matching the HTML example added in patternfly

• Update component tests if needed

• Update TypeScript types for new props

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Jira Issue: PF-3735

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Wizard - glass style follow up #12274

🔗 Related PRs

#12140 - feat(TreeView): add support for disabled TreeViewListItems [merged]
#12197 - chore(miscellaneous): update demo flags from js to ts [merged]
#12198 - chore(miscellaneous): update demo and example flags from js to ts [merged]
#12199 - chore(grid,stack): update examples flags from js to ts [merged]
#12208 - chore(wizard, date picker): update tags and examples from js to ts [merged]

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[kmcfaul]

Story points - 3