Document libgccjit++.h

Document libgccjit++.h

gcc/jit/ChangeLog:
	* docs/cp/index.rst: New file.
	* docs/cp/intro/index.rst: New file.
	* docs/cp/intro/tutorial01.rst: New file.
	* docs/cp/intro/tutorial02.rst: New file.
	* docs/cp/intro/tutorial03.rst: New file.
	* docs/cp/intro/tutorial04.rst: New file.
	* docs/cp/topics/contexts.rst: New file.
	* docs/cp/topics/expressions.rst: New file.
	* docs/cp/topics/functions.rst: New file.
	* docs/cp/topics/index.rst: New file.
	* docs/cp/topics/locations.rst: New file.
	* docs/cp/topics/objects.rst: New file.
	* docs/cp/topics/results.rst: New file.
	* docs/cp/topics/types.rst: New file.
	* docs/examples/tut01-hello-world.cc: New file.
	* docs/examples/tut02-square.c: Fix missing newline in output.
	* docs/examples/tut02-square.cc: New file.
	* docs/examples/tut03-sum-of-squares.cc: New file.
	* docs/examples/tut04-toyvm/toyvm.cc: New file.
	* docs/index.rst: Move summary to above the table of contents.
	Add text about the C vs C++ APIs.
	* docs/topics/contexts.rst: Fix a typo.

	* docs/_build/texinfo/libgccjit.texi: Regenerate.
	* docs/_build/texinfo/factorial1.png: New file.
	* docs/_build/texinfo/sum-of-squares1.png: New file.

From-SVN: r218588