fix(ci): add RTD config file

[jeertmans]

EDIT: that would be great to enable RTD builds on PRs, to check if my fix is working or not :-)

---

Hello,

RTD builds are broken because they need some config file.
I just copy/pasted some RTD config file from a personal project that happens
to use jupyter_rfb and VisPy too in the documentation.

Closes #96

[almarklein]

Let's merge and see what rtd does ...

[almarklein]

EDIT: that would be great to enable RTD builds on PRs, to check if my fix is working or not :-)

Good point. I read it too late 😅

[jeertmans]

Maybe you can revert the changes and re-open this PR? Because, apparently, the builds fail, seehttps://readthedocs.org/api/v2/build/24668980.txt

[almarklein]

Building wheels for collected packages: jupyter_rfb
  Building wheel for jupyter_rfb (pyproject.toml): started
  Building wheel for jupyter_rfb (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error
  
  × Building wheel for jupyter_rfb (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [50 lines of output]
      running bdist_wheel
      running jsdeps
      `yarn` unavailable.  If you're running this command using sudo, make sure `yarn` is available to sudo
      Traceback (most recent call last):
        File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-rfb/envs/latest/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
          main()
        File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-rfb/envs/latest/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/home/docs/checkouts/readthedocs.org/user_builds/jupyter-rfb/envs/latest/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 251, in build_wheel
          return _build_backend().build_wheel(wheel_directory, config_settings,
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/build_meta.py", line 410, in build_wheel
          return self._build_with_temp_dir(
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/build_meta.py", line 395, in _build_with_temp_dir
          self.run_setup()
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/build_meta.py", line 311, in run_setup
          exec(code, locals())
        File "<string>", line 92, in <module>
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/__init__.py", line 103, in setup
          return distutils.core.setup(**attrs)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/core.py", line 184, in setup
          return run_commands(dist)
                 ^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/core.py", line 200, in run_commands
          dist.run_commands()
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/dist.py", line 969, in run_commands
          self.run_command(cmd)
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/dist.py", line 968, in run_command
          super().run_command(command)
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/dist.py", line 988, in run_command
          cmd_obj.run()
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/jupyter_packaging/setupbase.py", line 503, in run
          [self.run_command(cmd) for cmd in cmds]
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/jupyter_packaging/setupbase.py", line 503, in <listcomp>
          [self.run_command(cmd) for cmd in cmds]
           ^^^^^^^^^^^^^^^^^^^^^
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/cmd.py", line 316, in run_command
          self.distribution.run_command(command)
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/dist.py", line 968, in run_command
          super().run_command(command)
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/setuptools/_distutils/dist.py", line 988, in run_command
          cmd_obj.run()
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/jupyter_packaging/setupbase.py", line 274, in run
          c.run()
        File "/tmp/pip-build-env-dyjv5dy6/overlay/lib/python3.11/site-packages/jupyter_packaging/setupbase.py", line 401, in run
          raise ValueError(('missing files: %s' % missing))
      ValueError: missing files: ['/home/docs/checkouts/readthedocs.org/user_builds/jupyter-rfb/checkouts/latest/js/dist/index.js']
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for jupyter_rfb
Failed to build jupyter_rfb
ERROR: Could not build wheels for jupyter_rfb, which is required to install pyproject.toml-based projects

I'll turn on rtd for pr's, so we can fix it in a single pr :)