[Website] Tidy use of and linking to Arrow logo

Now that ARROW-15684 is merged, it would make sense to be consistent in how the Arrow logo is used on the website, and to link to the Visual Identity page where appropriate (e.g., there is a natural place to link to it from the Powered By page). In addition to improving the cross-linking between pages, look for cases where the site can use the updated files rather than the outdated ones (but do not delete the outdated files)