Page sidebar - support for context selector/perspective switcher

[mcoker]

Follow up for core issue https://github.com/patternfly/patternfly/pull/6839

Pretty straight forward update - just adds a variant to .pf-v6-c-page__sidebar-body to add the modifier class .pf-m-context-selector. From there the user can pass a menu-toggle/menu.

I'm not sure if we have an existing demo for this anywhere but core has added one under page demos - https://patternfly-pr-6839.surge.sh/components/page/html-demos/context-selector-in-sidebar/