Use the logo URL for the C++ docs

It appears that

    readthedocs/sphinx_rtd_theme#1171

and

    sphinx-doc/sphinx#9381

have been released in their respective packages and we can use the logo url in our docs.

Author: tronical

api/cpp/docs/conf.py

@@ -103,7 +103,7 @@
 
 html_show_sourcelink = False
 
-html_logo = "../../logo/slint-logo-small-light.svg"  # relative to target/cppdocs!
+html_logo = "https://slint-ui.com/logo/slint-logo-small-light.svg"
 
 myst_enable_extensions = [
     "html_image",