Add documentation: inject, launcher and iidxhooks using them

doc/inject.md

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+# Inject
+
+Inject is a tool that creates a process for a specified application executable
+file and injects a specified list of library/dll files into the process before
+it is kicked off.
+
+Typically, the tool is run when you use one of the bat script files to start
+any of the games that require its usage, e.g. older IIDX games.
+
+It comes with further option switches for development, e.g. Halting until
+a debugger is attached, outputting debug output to a logfile etc.
+Just run the application without any parameters to get a usage message:
+```
+inject.exe
+```
+
+How inject.exe is used can be taken from the bat script files of various
+(typically older) games.