[docs] Warn about using withRoot HOC more than one time per page (#12692)
[docs] Warn about using withRoot HOC more than one time per page (#12692)

* [docs] Improve gatsby example giving more details on the HOC component

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