Blog post on bundles

[haydentherapper]

Description

With @steiza's most recent blog post on Cosign adding support for bundles and conformance testing, it would be great to have a blog post that describes the current state of bundles, including:

1. What's a protobuf bundle and the motivation behind its creation
2. What's a Cosign bundle and why we're moving away from those
3. Conformance across clients with accepting and outputting bundles

cc @codysoyland, could this be a precursor to your talk, ask readers to attend for more info?