Add yaml configuration examples to the "Custom Payment Gateway" cookbook

[DennisCodeBuds]

In my projects I tend to use YAML for the configuration files so I thought it might be helpful to others to add configuration examples in the YAML format

Q	A
Branch?	1.9, 1.10 or master
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets
License	MIT

[CoderMaggie]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "patch-1" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[CoderMaggie]

Thanks, @DennisCodeBuds! 🎉 Congrats on your first contribution to Sylius 💪🏼