Organize book logos into _static/logos directory

- Move logo-tinytorch-grey.png to _static/logos/
- Move logo-tinytorch-simple.png to _static/logos/
- Move logo-tinytorch-white.png to _static/logos/
- Move tensortorch.png to _static/logos/
- Update _config.yml to reference new logo path
- Keeps all logo versions organized in standard static assets location

book/_config.yml

@@ -4,7 +4,7 @@
 title: "TinyTorch"
 author: "Prof. Vijay Janapa Reddi (Harvard University)"
 copyright: "2025"
-logo: logo-tinytorch-white.png
+logo: _static/logos/logo-tinytorch-white.png
 
 # Book description and metadata
 description: >-