setup.cfg: add metadata.project_urls (#71)

* setup.cfg: add `metadata.project_urls`

Fix #69

According to a project example https://pypi.org/project/PyScaffold/4.3/ +
https://github.com/pyscaffold/pyscaffold/blob/v4.3/setup.cfg#L7-L15,
the "Homepage" link pulled from `metadata.url` apparently stays even if
I add `project_urls`, so all links at PyPI resulting from this change
should be (I'm not sure about their order, but again links on
https://pypi.org/project/PyScaffold/4.3/ seem to be sorted
alphabetically except the implicit "Homepage" link at the beginning):

- [Homepage](https://kaitai.io)
- [Gitter](https://gitter.im/kaitai_struct/Lobby)
- [Source](https://github.com/kaitai-io/kaitai_struct_python_runtime)
- [Tracker](https://github.com/kaitai-io/kaitai_struct_python_runtime/issues)
- [Twitter](https://twitter.com/kaitai_io)

Currently (see https://pypi.org/project/kaitaistruct/0.10/) the project
links are just:

- [Homepage](https://kaitai.io)

* setup.cfg: remove Twitter link

See #71 (review)

* setup.cfg: add Documentation link to doc.kaitai.io

setup.cfg

@@ -4,6 +4,11 @@ version = attr: kaitaistruct.__version__
 author = Kaitai Project
 author_email = greycat@kaitai.io
 url = https://kaitai.io
+project_urls =
+    Documentation = https://doc.kaitai.io/
+    Source = https://github.com/kaitai-io/kaitai_struct_python_runtime
+    Tracker = https://github.com/kaitai-io/kaitai_struct_python_runtime/issues
+    Gitter = https://gitter.im/kaitai_struct/Lobby
 description = Kaitai Struct declarative parser generator for binary data: runtime library for Python
 long_description = file: README.md
 long_description_content_type = text/markdown