changes to replace Section_start.txt

doc/src/Build.txt

doc/src/Build_basics.txt

doc/src/Build_cmake.txt

doc/src/Build_extras.txt

doc/src/Build_link.txt

doc/src/Build_make.txt

doc/src/Build_package.txt

doc/src/Build_settings.txt

doc/src/Install.txt

doc/src/Install_git.txt

doc/src/Install_linux.txt

doc/src/Install_mac.txt

doc/src/Install_patch.txt

doc/src/Install_svn.txt

doc/src/Install_tarball.txt

doc/src/Install_windows.txt

doc/src/Run.txt

doc/src/Run_basics.txt

doc/src/Run_options.txt

doc/src/Run_output.txt