Split sticky header demo into different files.

[m0ksem]

This is one big demo. https://vuestic.dev/en/ui-elements/data-table#sticky-header-and-footer

Might be three different demos, so code is easier to read.

[asvae]

I think we can remove most of these examples (leaving maybe only both sticky header and footer). All 4 are useful for demo, but useless for user.