main.star: add a logo via luci.milo()

https://chromium.googlesource.com/infra/luci/luci-go/+/refs/heads/main/lucicfg/doc/README.md#luci.milo

Bug: 1028422
Change-Id: I4d9c35efed302fb081f2135b84fd92bb43c3eb29
Reviewed-on: https://chromium-review.googlesource.com/c/codecs/infra/+/2941680
Reviewed-by: Jose Alanis Baez <alanisbaez@google.com>
Reviewed-by: James Zern <jzern@google.com>