Remove references to `swedbank-pay-design-guide-jekyll-theme` to break
the circular dependency between the two codebases.

Disable debug to avoid HTMLProofer loading awesome_print, because it
leads to the following failure:

```
Traceback (most recent call last):
	10: from /var/jekyll/entrypoint/lib/entrypoint.rb:3:in `<main>'
	 9: from /var/jekyll/entrypoint/lib/entrypoint.rb:3:in `require_relative'
	 8: from /var/jekyll/entrypoint/lib/commander.rb:7:in `<top (required)>'
	 7: from /var/jekyll/entrypoint/lib/commander.rb:7:in `require_relative'
	 6: from /var/jekyll/entrypoint/lib/commands/verifier.rb:4:in `<top (required)>'
	 5: from /var/jekyll/entrypoint/lib/commands/verifier.rb:4:in `require'
	 4: from /usr/gem/ruby/2.7.0/gems/html-proofer-4.4.0/lib/html-proofer.rb:3:in `<top (required)>'
	 3: from /usr/gem/ruby/2.7.0/gems/html-proofer-4.4.0/lib/html-proofer.rb:3:in `require_relative'
	 2: from /usr/gem/ruby/2.7.0/gems/html-proofer-4.4.0/lib/html_proofer.rb:18:in `<top (required)>'
	 1: from /usr/gem/ruby/2.7.0/gems/zeitwerk-2.6.0/lib/zeitwerk/kernel.rb:35:in `require'
```

https://github.com/SwedbankPay/jekyll-plantuml-docker/actions/runs/3079379097/jobs/4975588911#step:5:2106

- Fix the RuboCop error:
  lib/commands/verifier.rb:78:21: C: [Corrected] Style/WordArray:
  Use %w or %W for an array of words.
- Add rubocop-rake and rubocop-rspec to rubocop config.

See 60ca6c7 for an explanation.