Fix sidebar logo not loading in HTML docs (#12939)

The mypy logo wasn't loading on https://mypy.readthedocs.io/ because
it's referenced via an http:// (not https) URL. The logo's URL is
http://mypy-lang.org/static/mypy_light.svg; that server is not
accessible via https, so I've moved mypy_light.svg into docs/source and
referenced it from there.

The project README also referenced the logo via the same URL, so for
consistency, the README now points at the same logo file in docs/source.
(Although the README's logo did load because GitHub proxies external
resources referenced in markdown files).

Author: h4l

README.md

@@ -1,4 +1,4 @@
-<img src="http://mypy-lang.org/static/mypy_light.svg" alt="mypy logo" width="300px"/>
+<img src="docs/source/mypy_light.svg" alt="mypy logo" width="300px"/>
 
 Mypy: Static Typing for Python
 =======================================

docs/source/conf.py

@@ -122,7 +122,7 @@
 
 # The name of an image file (relative to this directory) to place at the top
 # of the sidebar.
-html_logo = "http://mypy-lang.org/static/mypy_light.svg"
+html_logo = "mypy_light.svg"
 
 # The name of an image file (within the static path) to use as favicon of the
 # docs.  This file should be a Windows icon file (.ico) being 16x16 or 32x32