A new theme for the kitty website

Supports dark mode. Also re-organize the documentation
a bit, making it more hierarchical.

Have a nicer landing page

docs/quickstart.rst

@@ -0,0 +1,28 @@
+.. _quickstart:
+
+Quickstart
+===========
+
+.. toctree::
+   :hidden:
+
+   binary
+   build
+
+Pre-built binaries of |kitty| are available for both macOS and Linux.
+See the :doc:`binary install instructions </binary>`. You can also
+:doc:`build from source </build>`.
+
+Additionally, you can use your favorite package manager to install the |kitty|
+package, but note that some Linux distribution packages are woefully outdated.
+|kitty| is available in a vast number of package repositories for macOS
+and Linux.
+
+.. image:: https://repology.org/badge/tiny-repos/kitty.svg
+   :target: https://repology.org/project/kitty/versions
+   :alt: Number of repositories kitty is available in
+
+See :doc:`Configuring kitty <conf>` for help on configuring |kitty| and
+:doc:`Invocation <invocation>` for the command line arguments |kitty| supports.
+
+For a tour of kitty's design and features, see the :doc:`/overview`.