make logo come from AEP content repo rather than site generator

[toumorokoshi]

The logo in the top-left of the site currently comes from the site generator repo. It'd be great if the logo could come from the content repo instead (e.g. google.aip.dev). For example, maybe the presence of a logo.svg in that repo could override the default Google logo.

The current version of the AEP repo just ignores the logo, but it would be better to just source it from the source repo instead.