chore(ci): add yaml linter workflow and fix yaml formatting

[qdm12]

Why this should be merged

To have consistent, warning-free yaml files, enforced by the CI

How this works

• New CI job triggering only on yaml/yml file modifications using the built-in yamllint with a .github/yamllint.yml configuration
• Fix existing yml files to pass CI
• Ignore geth original files such as .travis.yml files

How this was tested

New CI job passing

[ARR4N]

Out of interest, why are these two explicitly required?

[qdm12]

Just so the (non required) workflow only triggers when modifying .yml/.yaml files. I suppose the workflow is yml too, so no need to specify it, but I really added it out of habit without thinking this through 😄 Although thinking about it, it might be worth leaving it in case someone copies the workflow (i.e. to make a markdown one) so he/she doesn't forget to have a trigger file path on the workflow file itself.

[ARR4N]

Interesting! How did it know that these fields exist?! I've made suggestions with reasons so my original rationale is now explicit.

[qdm12]

It didn't, I looked it up because the linter was complaining about the empty object, i.e.

- blabla:
- abc:

is not making the linter happy, so I've set just one field to its zero value 🤷 But good you've added a reason for each, even better 👍