Stable version bump (0.8 -> 0.9) needed in docs

[redbrogdon]

A number of pages in the docs still list A2UI v0.8 as the latest stable release and v0.9 as a draft. These pages should be updated to show 0.9 as stable an 0.10 as draft.

Examples:

• https://a2ui.org/
• https://a2ui.org/specification/v0.8-a2ui/