Simplify doc build

[germa89]

There is a chunk of code which seems unnecesary:

pymapdl/src/ansys/mapdl/core/launcher.py

Lines 1576 to 1604 in 3f16bf6

	if pymapdl.BUILDING_GALLERY: # pragma: no cover
	LOG.debug("Building gallery.")
	# launch an instance of pymapdl if it does not already exist and
	# we're allowed to start instances
	if GALLERY_INSTANCE[0] is None:
	mapdl = launch_mapdl(
	start_instance=True,
	cleanup_on_exit=False,
	loglevel=loglevel,
	set_no_abort=set_no_abort,
	**start_parm,
	)
	GALLERY_INSTANCE[0] = {"ip": mapdl._ip, "port": mapdl._port}
	return mapdl
	# otherwise, connect to the existing gallery instance if available
	elif GALLERY_INSTANCE[0] is not None:
	mapdl = MapdlGrpc(
	ip=GALLERY_INSTANCE[0]["ip"],
	port=GALLERY_INSTANCE[0]["port"],
	cleanup_on_exit=False,
	loglevel=loglevel,
	set_no_abort=set_no_abort,
	use_vtk=use_vtk,
	**start_parm,
	)
	if clear_on_connect:
	mapdl.clear()
	return mapdl

This does not need to be in the library.

We can just use Sphinx events to spawn an MAPDL instance.

The pseudo code is as follow:

def spawn_mapdl_instance(app, docname, source):
     app._mapdl = launch_mapdl()  # it should account for local and remote.

def kill_mapdl_instance(app, exception):
    app._mapdl.exit()

def setup(app):
    app.connect('builder-inited', spawn_mapdl_instance)
    app.connect('build-finished', kill_mapdl_instance)

This way, we can remove the mentioned code from the library itself. It has been long irritating my eyes.

Reference: https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx-core-events

[germa89]

Related to this #2922