Use a relative path for docs index page logo

Changes the paths for the logo on the index page of the documentation
to use relative links instead of absolute ones.

The current absolute path is just fine as long as the site
is not hosted under another directory. When opening files locally
(without serving them in docfx) these images will fail to load.
In addition, if these files are served under a directory that is
not the root endpoint (like: `docs.com/stable/`) the images will
also break.

I tested these changes locally using these steps:

- Build & serve docs: `docfx docs/docfx.json --serve`
- Verify that logo shows up on index page using all 3 themes
- Open `docs/_site/index.html`
- Verify logo works on all 3 themes

docs/_template/light-dark-theme/styles/dark.css

@@ -304,5 +304,5 @@ span.arrow-r{
 }
 
 .logo-switcher {
-    background: url("/marketing/logo/SVG/Combinationmark White.svg") no-repeat;
+    background: url("../marketing/logo/SVG/Combinationmark White.svg") no-repeat;
 }

docs/_template/light-dark-theme/styles/gray.css

@@ -311,5 +311,5 @@ span.arrow-r{
 }
 
 .logo-switcher {
-    background: url("/marketing/logo/SVG/Combinationmark White.svg") no-repeat;
+    background: url("../marketing/logo/SVG/Combinationmark White.svg") no-repeat;
 }

docs/_template/light-dark-theme/styles/light.css

@@ -113,5 +113,5 @@ span.arrow-r{
 }
 
 .logo-switcher {
-    background: url("/marketing/logo/SVG/Combinationmark.svg") no-repeat;
+    background: url("../marketing/logo/SVG/Combinationmark.svg") no-repeat;
 }