Restore logo to ReadTheDocs pages

Adding the config file (.readthedocs.yml) got our ReadTheDocs site building again, after being broken for a while. However, it is now created using Sphinx 7.1.2 and Press Theme 0.9.1 and that has broken some things. Most prominently, the OCIO logo in the top left corner is gone and replaced with the word "logo". There may be other stuff broken too.

To see the issue, use the menu in the lower right corner of the RTD pages to switch from "v: latest" to an earlier version.

EDIT: Per the comments below, the logo does seem to be loading now. But for people running in "dark mode", it's a black logo against an almost black screen. We should be using the magenta OCIO logo since it will be visible in either light or dark mode.

If people notice any other aesthetic issues likely due to the infrastructure version updates, please note them in the comments below.