Ignore feature flags for `yamllint`

What does this MR do and why?

Fixes lefthook failure on git push. Validation fails for content/_data/feature_flags.yaml that is dynamically generated by rake generate_feature_flags. CI tests don't cover this.

Fixing generator to produce valid YAML is hard to impossible https://github.com/ruby/psych/issues/591 so we're just ignoring this.

Screenshots, screen recordings, or links to review app

INFO: Running yamllint tests...
content/_data/feature_flags.yaml
  4:3       error    wrong indentation: expected 4 but found 2  (indentation)
  48:23     error    trailing spaces  (trailing-spaces)
  69:23     error    trailing spaces  (trailing-spaces)
  96:23     error    trailing spaces  (trailing-spaces)
  153:23    error    trailing spaces  (trailing-spaces)
  202:23    error    trailing spaces  (trailing-spaces)
  258:23    error    trailing spaces  (trailing-spaces)

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Make sure lefthook is installed.
3. Run git push or

lefthook run pre-push

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.