[docs] Fix missing ponctuation on descriptions

[oliviertassinari]

Make the next docs-infra upgrade easier for https://github.com/mui/material-ui/pull/44292.

[mui-bot]

Deploy preview: https://deploy-preview-15229--material-ui-x.netlify.app/

Updated pages:

• docs/data/data-grid/joy-ui/joy-ui.md
• docs/data/date-pickers/custom-field/custom-field.md
• docs/data/date-pickers/custom-layout/custom-layout.md
• docs/data/date-pickers/experimentation/experimentation.md

Generated by :no_entry_sign: dangerJS against 1dd4265e31c05cf1d85a7620e27a00b0c27831a2