doc: fix figure numbering in graph guide

doc: fix figure numbering in graph guide

Some figures had a title inside the picture but not in RST file.
As a consequence, some versions of Sphinx are emitting a warning.

	Warning, treated as error:
	doc/guides/prog_guide/graph_lib.rst:64:
	no number is assigned for figure: figure-anatomy-of-a-node

The titles are moved from SVG to RST,
except for graph_mem_layout.svg where in-picture title must be kept.

Fixes: 4dc6d8e63c16 ("doc: add graph library guide")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Jerin Jacob <jerinj@marvell.com>