[ai-assistant] Make it clear that the API key must be private

[oliviertassinari]

The key is used directly for billing; if it's public, people can grab a random one and use it on their own server. Until we take care of https://github.com/mui/mui-private/issues/1057. I believe we can't, in the docs, tell people to "avoid", it should be "don't. I have been wanting to fix this for a long time, #19142 gave more importance to it.

[mui-bot]

Deploy preview: https://deploy-preview-19244--material-ui-x.netlify.app/

Updated pages:

• docs/data/data-grid/ai-assistant/ai-assistant.md

Bundle size report

Bundle	Parsed size	Gzip size
@mui/x-data-grid	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-pro	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-premium	0B(0.00%)	🔺+2B(0.00%)
@mui/x-charts	0B(0.00%)	0B(0.00%)
@mui/x-charts-pro	0B(0.00%)	🔺+1B(0.00%)
@mui/x-date-pickers	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers-pro	0B(0.00%)	0B(0.00%)
@mui/x-tree-view	0B(0.00%)	0B(0.00%)
@mui/x-tree-view-pro	0B(0.00%)	▼-1B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against c7faa89

[oliviertassinari]

I have added more options on how we could simplify the DX in the future: https://github.com/mui/mui-private/issues/1057#issuecomment-3208302860. cc @arminmeh for awareness (no actions needed)