Plugins: add leaftlet-custom-searchbox (#5363)
Plugins: add leaftlet-custom-searchbox (#5363)

Extended the "search & popup" section with the details of the new "leaftlet-custom-searchbox" plugin.
iZucken/Leaflet,Ubisense/Leaflet,stefanocudini/Leaflet,msiadak/Leaflet,Ubisense/Leaflet,frogcat/Leaflet,scaddenp/Leaflet,stefanocudini/Leaflet,Leaflet/Leaflet,geoloep/Leaflet,msiadak/Leaflet,iZucken/Leaflet,CapeSepias/Leaflet,Ubisense/Leaflet,frogcat/Leaflet,msiadak/Leaflet,Leaflet/Leaflet,codeator/Leaflet,stefanocudini/Leaflet,CapeSepias/Leaflet,scaddenp/Leaflet,codeator/Leaflet,codeator/Leaflet,geoloep/Leaflet,geoloep/Leaflet,Leaflet/Leaflet,CapeSepias/Leaflet,scaddenp/Leaflet,frogcat/Leaflet,iZucken/Leaflet