add new features to mergify config

[jxs]

Issue Addressed

While addressing an issue on the rust-libp2p mergify.yml config file, finally had time to read its doc and suggest some nice things it has there to the lighthouse repo:

• automatically notify PR authors when their PR has conflicts with the target PR branch
• add the ability to PR's to the mergify queue by adding the add-to-queue label instead of having to reply to the PR with "@Mergifyio queue"
• add the ability to merge trivial PRs without other approvals by adding the trivial label

Notes

• the lighthouse team doesn't exist yet, we can create it or use the sigp team which has all its members
• took the opportunity to also change the merged commit message by instead of having the commits having the PR description sections, tell me wdyt @jimmygchen, @michaelsproul

[CLAassistant]

All committers have signed the CLA.

[chong-he]

May need to change the target branch to unstable?

[jxs]

May need to change the target branch to unstable?

Hi Ck, since this affects CI needs to be merged against stable