Update docs.md
Update docs.md

`config.pages.theme` does not exist and is empty. This probably should be `config.system.pages.theme`
getgrav/grav-learn,Quy/grav-learn,miljan-aleksic/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,Quy/grav-learn,miljan-aleksic/grav-learn,zikkuratvk/grav-learn,getgrav/grav-learn,getgrav/grav-learn,zikkuratvk/grav-learn,zikkuratvk/grav-learn,getgrav/grav-learn