rtd: add manimpango wheels (#1067)

see ManimCommunity/ManimPango#27 wheels are removed in the 
the latest release from PyPI so wheel is added manually so RTD doesn't fail.

Author: naveen521kk

docs/rtd-requirements.txt

@@ -1,2 +1,3 @@
 imageio-ffmpeg
-jupyterlab
+jupyterlab
+https://github.com/ManimCommunity/ManimPango/releases/download/v0.2.4/ManimPango-0.2.4-cp38-cp38-manylinux2014_x86_64.whl