transaction diagram was remade using graphviz
removing trailing spaces
Update Transaction State Diagram

add 2 more API hooks, TS_HTTP_POST_REMAP_HOOK & TS_HTTP_PRE_REMAP_HOOK,
into state diagram.
TS_HTTP_PRE_REMAP_HOOK is new name of
TS_HTTP_READ_REQUEST_PRE_REMAP_HOOK since v2.1.5
TS-2397: correct formatting and fix links

add the graphviz extension to conf.py
indent the digraph so it is actually picked up by sphinx
give it a nicer name, 'g' is too generic! Give it a nicer alt text,
because otherwise we end up with the digraph code.
Consistent formatting & links.

Outstanding: TS_HTTP_REQUEST_TRANSFORM_HOOK and
TS_HTTP_RESPONSE_TRANSFORM_HOOK still need to link to the correct
constants in doc/reference