kernel-shark: Separate trace-cmd and kernelshark documentations.

KernelShark is going to be a stand alone package, separate from trace-cmd.
The code is already in its own directory, but HTML and man page documents
are still not separated.
All KernelShark related documentation (HTML and the man page) is moved
from Documentation/ to kernel-shark/Documentation/ directory.
New targets are added to trace-cmd Makefile:
 doc_gui - build KernelShark documentation
 doc_gui_clean - clean KernelShark documentation
 install_doc_gui - install KernelShark documentation
The existing Makefile targets:
 doc, doc_clean, install_doc
are modified to work only with trace-cmd documentation.
The new Makefile targets must be used to build and install KernelShark docs.

This change was requested by Bas van Dijk <v.dijk.bas@gmail.com>, to
simplify packaging trace-cmd and kernelshark for nixpkgs, as two
different packages.

The existing kernel-shark/doc directory is used when doxygen documentation
is generated. All doxygen related files - doxygen config and doxygen output
files are moved to kernel-shark/Documentation/doxygen.

Link: http://lore.kernel.org/linux-trace-devel/20190806123727.25705-1-tz.stoyanov@gmail.com

Acked-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>