Simplify doc build

[German]

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

[German]

Related to this #2922