Add graphs of the overall architecture

[Andrea Crotti]

Adding to the documentation some graphs in graphviz format.

[Andrea Crotti (andrea-crotti-0)]

Running sphinx in the "default" way fails, because the destination directory doesn't exist, and readthedocs for example fails to build the documentation.

WARNING: "latex_documents" config value references unknown document index
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
  File "/home/docs/sites/readthedocs.org/checkouts/readthedocs.org/user_builds/gnu-mailman/checkouts/latest/conf.py", line 225, in index_html
    os.chdir('build/sphinx/html')
OSError: [Errno 2] No such file or directory: 'build/sphinx/html'
Error in sys.exitfunc:
Traceback (most recent call last):
  File "/usr/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
  File "/home/docs/sites/readthedocs.org/checkouts/readthedocs.org/user_builds/gnu-mailman/checkouts/latest/conf.py", line 225, in index_html
    os.chdir('build/sphinx/html')
OSError: [Errno 2] No such file or directory: 'build/sphinx/html'

[Sumana Harihareswara (sumanah)]

Maybe we could reuse diagrams from http://aosabook.org/en/mailman.html .

[Abhilash Raj (raj-abhilash1)]

This bug has been moved to the new gitlab repo here: https://gitlab.com/mailman/mailman/issues/35