Fix logo relative path

Fix broken `html_logo` image path in navbar when a page is in a subdirectory, by making the logo path relative to the page location with `pathto()`.

I've set my `html_logo` setting in conf.py to a PNG image in the `_static` directory. It works for the pages that are in the root of the Sphinx project folder, but I have subdirectories under it, and when I navigate to a page in a subdirectory, the logo disappears from the navbar.

Looking at the HTML source, I see that for pages in subdirectories, the `img src=` still points to `_static/myimage.png` but `_static` is in the root, so it needs to be made relative to the page location and should be something like `../_static/myimage.png` or `../../_static/myimage.png`, etc.