Add yaml linting

[sed-i]

Issue

YAML files aren't linted.

Solution

Lint YAML files.

Context

Using yamllint may be obviated when:

• charmcraft analyze is fixed -- Add charmcraft analyze to linting observability#21
• rule validation is implemented -- feature(ci): validate alert rules observability#22

Testing Instructions

Release Notes

Add yaml linting.

[sed-i]

Seems like this could be an argument in favor of a custom "generic o11y charm" pre-commit hook: instead of opening PRs on all repos, could have added yamllint just in one place.

[sed-i]

#21 and #22 already capture all we need. Closing.