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/unscroll.rst

@@ -0,0 +1,34 @@
+.. _unscroll:
+
+Unscrolling the screen
+========================
+
+This is a small extension to the `SD (Pan up) escape code
+<https://vt100.net/docs/vt510-rm/SD.html>`_ from the VT-420 terminal. The
+``SD`` escape code normally causes the text on screen to scroll down by the
+specified number of lines, with empty lines appearing at the top of the screen.
+This extension allows the new lines to be filled in from the scrollback buffer
+instead of being blank.
+
+The motivation for this is that many modern shells will show completions in a
+block of lines under the cursor, this causes some of the on-screen text to be
+lost even after the completion is completed, because it has scrolled off
+screen. This escape code allows that text to be restored.
+
+If the scrollback buffer is empty or there is no scrollback buffer, such as for
+the alternate screen, then the newly inserted lines must be empty, just as with
+the original ``SD`` escape code. The maximum number of lines that can be
+scrolled down is implementation defined, but must be at least one screen worth.
+
+The syntax of the escape code is identical to that of ``SD`` except that it has
+a trailing ``+`` modifier. This is legal under the `ECMA 48 standard
+<https://www.ecma-international.org/publications-and-standards/standards/ecma-48/>`_
+and unused for any other purpose as far as I can tell. So for example, to
+unscroll three lines, the escape code would be::
+
+    CSI 3 + T
+
+See `discussion here
+<https://gitlab.freedesktop.org/terminal-wg/specifications/-/issues/30>`_.
+
+.. versionadded:: 0.20.2