docs(VDataTable): clarify that height is required with fixed-header

[sridhar-3009]

Fixes #20201

The fixed-header prop requires a height constraint to work. Without it, the table expands to show all rows and no scrolling occurs, so the header cannot be sticky. This was undocumented for v-data-table.

Added:

• A "Fixed header" section to data-tables/basics.md with an info callout explaining the height requirement
• A new prop-fixed-header.vue example showing the correct combination of props

[J-Sek]

Would be nice to cover fixed-footer in the same example. Otherwise it is more of a bloat/noise.

[sridhar-3009]

Added a Fixed footer section with a prop-fixed-footer example covering fixed-footer in the same style as the fixed-header example above it.