Update lightning logo (Lightning-AI#4490)

* docs logo * use png * cleaning * vectorial * icon * icon * use png * cleaning * aync Co-authored-by: Jirka Borovec <jirka@pytorchlightning.ai> Co-authored-by: Sean Naren <sean.narenthiran@gmail.com> Co-authored-by: chaton <thomas@grid.ai>

Author: 4 people