[Docs] Remove kapa AI button (#2849)

Removes kapa AI button from the Playground doc site – it triggers a
React error
[here](https://wordpress.github.io/wordpress-playground/api/blueprints/interface/EnableMultisiteStep/#Index):

<img width="2786" height="1226" alt="CleanShot 2025-11-03 at 11 07
10@2x"
src="https://github.com/user-attachments/assets/0ed25935-012c-4664-b95f-ecbb845ae1aa"
/>

This is a temporary measure to fix the documentation page until the
error is figured out—assuming we continue with Kapa AI (it's a free
trial).

This PR also updates the Docusaurus version from `3.8.1` to `3.9.2`. It
doesn't seem strictly necessary but was a low-hanging fruit to co-locate
with this PR.

## Testing instructions

Build the doc site locally, confirm there's no error at
`/wordpress-playground/api/blueprints/interface/EnableMultisiteStep/#Index`

Author: fellyph