Add documentation into the test suite

[SanderElias]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

During test we only use the sample-blog app to run tests against

Issue Number: N/A

What is the new behavior?

All root level markdown files in the docs folder will be tested for being generated, and being similar. This means, when you update some docs, you might need to update the snapshots

Does this PR introduce a breaking change?

• Yes
• No

Other information