docs: generate deltachat-rpc-client documentation

To preview the docs, run:
```
scripts/build-python-docs.sh
firefox dist/html/index.html
```

I have removed the Makefile because modern Sphinx Makefile is just a
wrapper for `sphinx-build -M`:
3596590317/sphinx/templates/quickstart/Makefile.new_t
and sphinx-quickstart even has an option `--no-makefile`.
`make.bat` makes even less sense.
In `scripts/build-python-docs.sh` I use `sphinx-build` directly
without `make` wrapper.

python/doc/jsonrpc/develop.rst

@@ -0,0 +1,68 @@
+===========
+Development
+===========
+
+To develop JSON-RPC bindings,
+clone the `deltachat-core-rust <https://github.com/deltachat/deltachat-core-rust/>`_ repository::
+
+   git clone https://github.com/deltachat/deltachat-core-rust.git
+
+Testing
+=======
+
+To run online tests, set ``CHATMAIL_DOMAIN``
+to a domain of the email server
+that can be used to create testing accounts::
+
+    export CHATMAIL_DOMAIN=nine.testrun.org
+
+Then run ``scripts/run-rpc-test.sh``
+to build debug version of ``deltachat-rpc-server``
+and run ``deltachat-rpc-client`` tests
+in a separate virtual environment managed by `tox <https://tox.wiki/>`_.
+
+Development Environment
+=======================
+
+Creating a new virtual environment
+to run the tests each time
+as ``scripts/run-rpc-test.sh`` does is slow
+if you are changing the tests or the code
+and want to rerun the tests each time.
+
+If you are developing the tests,
+it is better to create a persistent virtual environment.
+You can do this by running ``scripts/make-rpc-testenv.sh``.
+This creates a virtual environment ``venv`` which you can then enter with::
+
+   . venv/bin/activate
+
+Then you can run the tests with
+
+::
+
+    pytest deltachat-rpc-client/tests/
+
+Refer to `pytest documentation <https://docs.pytest.org/>` for details.
+
+If make the changes to Delta Chat core
+or Python bindings, you can rebuild the environment by rerunning
+``scripts/make-rpc-testenv.sh``.
+It is ok to rebuild the activated environment this way,
+you do not need to deactivate or reactivate the environment each time.
+
+Using REPL
+==========
+
+Once you have a development environment,
+you can quickly test things in REPL::
+
+   $ python
+   >>> from deltachat_rpc_client import *
+   >>> rpc = Rpc()
+   >>> rpc.start()
+   >>> dc = DeltaChat(rpc)
+   >>> system_info = dc.get_system_info()
+   >>> system_info["level"]
+   'awesome'
+   >>> rpc.close()