Rename nav items in alignment with research findings

[edonehoo]

The design solution indicated some changes to existing nav item labels. Update these in the filenames, file metadata, and in embedded links.

The following naming changes are recommended:

Before	After
Upgrade	Upgrade guide
Icons (design foundations)	Iconography
Design foundations	Foundations and styles
UX writing	Content design
Accessibility scorecard	Product scorecard
Design for accessibility	Design
Develop for accessibility	Develop
Testing your accessibility	Test your product
PatternFly AI	AI
Usage and behavior	Component usage and behavior
Tokens	Design tokens
Developer resources	Developer guides

Moving files to make new top-level items:
About us
Releases (folder with release highlights and upgrade guide)

related prs in other repos:

• react-component-groups: Move within Extensions
• react-topology: Move within Extensions
• chatbot: Move within Extensions
• patternfly-react: Move OUIA from dev resources to dev guides
• patternfly: Move utility classes and layouts to be within Foundations and styles