Docs: Move build instruction and user guide into technical reference

Move build instruction document and user guide document from getting
started into technical references folder. Rename 'tfm_user_guide.rst'
to 'tfm_run_instruction.rst'. This will make the document structure
more clear for readers.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I69d46151f2cb97c828c1b82775ffd1af9032ed45