docs: conf.py: Add zulip logo to the sidebar.

This commit adds the Zulip logo to the readthedocs
documentation by adding an .svg file (taken from
zulip.com) to the `docs/images` directory.
Morever, it removes the name `Zulip` which was
written at the top of the sidebar because the
logo already has that.

docs/conf.py

@@ -124,6 +124,7 @@ else:
 # documentation.
 html_theme_options = {
     'collapse_navigation': False,
+    'logo_only': True
 }
 
 # Add any paths that contain custom themes here, relative to this directory.
@@ -138,7 +139,7 @@ html_theme_options = {
 
 # The name of an image file (relative to this directory) to place at the top
 # of the sidebar.
-#html_logo = None
+html_logo = 'images/zulip-logo.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