[docs] Improve CSS Grid documentation (#15477)
[docs] Improve CSS Grid documentation (#15477)

* Better (?) CSS Grid documentation

As discussed in #9513 starting at https://github.com/mui-org/material-ui/issues/9513#issuecomment-486299939 the current documentation was a little confusing and not really located in a logical place.

I'm not sure there *is* a logical place for this information, but it seems like the end of the document is better than in the middle of information talking about grid stuff that MUI *does* support.

I made the fact that Material UI doesn't provide any CSS Grid functionality explicit.

I also took out the warning about CSS Grid only being supported in the latest browsers.  As of right now it has 90% browser coverage at https://caniuse.com/#feat=css-grid.

* Update grid.md

kybarg/material-ui,oliviertassinari/material-ui,mbrookes/material-ui,mbrookes/material-ui,callemall/material-ui,rscnt/material-ui,rscnt/material-ui,allanalexandre/material-ui,callemall/material-ui,kybarg/material-ui,callemall/material-ui,kybarg/material-ui,oliviertassinari/material-ui,mui-org/material-ui,mui-org/material-ui,mui-org/material-ui,mbrookes/material-ui,rscnt/material-ui,lgollut/material-ui,oliviertassinari/material-ui,callemall/material-ui,kybarg/material-ui