Merge branch 'master' of https://root.cern.ch/git/root

bindings/pyroot/src/Adapters.cxx

bindings/pyroot/src/Converters.cxx

bindings/pyroot/src/Executors.cxx

bindings/pyroot/src/Pythonize.cxx

bindings/pyroot/src/RootWrapper.cxx

bindings/pyroot/src/TConstructorHolder.cxx

bindings/pyroot/src/TMethodHolder.cxx

cmake/modules/FindXROOTD.cmake

cmake/modules/SearchInstalledSoftware.cmake

docbook/users-guide/ALittleC++.md

docbook/users-guide/FittingHistograms.md

docbook/users-guide/GettingStarted.md

docbook/users-guide/Graphs.md

docbook/users-guide/Histograms.md

docbook/users-guide/Introduction.md

docbook/users-guide/Preface.md

docbook/users-guide/Trees.md

docbook/users-guide/pictures/02000030.png

docbook/users-guide/pictures/02000031.png

docbook/users-guide/pictures/02000032.png