[docs] Add focus highlight in composition snipets

[alexfauquette]

[mui-bot]

Deploy preview: https://deploy-preview-20614--material-ui-x.netlify.app/

Updated pages:

• docs/data/charts/bars/bars.md
• docs/data/charts/lines/lines.md
• docs/data/charts/pie/pie.md
• docs/data/charts/scatter/scatter.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%)	0B(0.00%)
@mui/x-charts	0B(0.00%)	0B(0.00%)
@mui/x-charts-pro	0B(0.00%)	0B(0.00%)
@mui/x-charts-premium	0B(0.00%)	0B(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%)	0B(0.00%)

Details of bundle changes

Generated by :no_entry_sign: dangerJS against 09c1e678faf29e7d0079efa001b16d928d1f4843

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #20614 will not alter performance

_{Comparing alexfauquette:document-focus-composition (09c1e67) with master (06e8498)[^unexpected-base]} [^unexpected-base]: No successful run was found on master (d5565ef) during the generation of this report, so 06e8498 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report.

Summary

✅ 13 untouched