Makefile: add a help target

Like:

swh-web$ make help
build-webpack-dev              Build webpack (bundle js assets)
build-webpack-test             | same with coverage activated
build-webpack-dev-no-verbose   | same as above without any output
build-webpack-prod             | build assets minified and with mappings for sentry
run-migrations-dev             Run django db migration (dev: swh.web.settings.development)
run-migrations-prod            | same with prod settings (swh.web.settings.production)
run-migrations-test            | same with test settings (swh.web.settings.tests)
add-users-test                 Create default django users (tests settings)
add-users-dev                  | same dev settings
add-users-prod                 | same using prod settings
clear-memcached                Clear locally running memcache (on localhost:1211)
run-django-webpack-devserver   Start a yarn & django servers from src in dev mode
run-django-webpack-dev         Build assets & start django from src using dev settings
run-django-webpack-prod        | same with prod settings
run-django-server-dev          Start django from src using dev settings
run-django-server-prod         | same with prod settings
run-gunicorn-server            Clear memcache and start django from gunicorn (prod settings)
run-django-webpack-memory-storages Start django from tests settings (using in-memory storages)
run-mirror-demo                Start django from tests config using a mirror setup
test-full                      Run all python tests
test-frontend                  Run cypress non-slow tests (no GUI)
test-frontend-full             | same, including slow tests
test-frontend-ui               Run cypress non-slow tests in a browser (GUI)
check-mypy                     Run mypy (with properly set env vars for django)