Rename gitbook.yaml to .gitbook.yaml

[unfulvio-godaddy]

Summary

201 added gitbook.yaml but this was intended to be a dot file .gitbook.yaml - my bad for not noticing this: currently this file does nothing because it's not parsed by GitBook until it has the correct format.

Reviewer checklist

- [x] Code changes review - [ ] Documentation changes review - [x] Unit tests pass - [x] Static analysis passes

[coveralls]

Coverage: 36.749%. Remained the same when pulling 8b0edda19d44ff35fe089fb624403c00aeb0cdb1 on chore/rename-gitbook-file into 2d3b0fc9654ef0db485ecd6a09ae066f5c0f0951 on trunk.

[unfulvio-godaddy]

@jeffpaul this wasn't added to a milestone because documentation isn't part of a release, and changes are/were instant, so it didn't belong to a release