feat(docs): skip markdown files that lack attributes

[kanadgupta]

🚥 Fixes #539

🧰 Changes

This updates our docs/changelogs/custompages commands to skip over any Markdown files that lack YAML frontmatter attributes.

Of all of the requests we get for docs syncing improvements (more are in the pipeline, we promise!), this one is by far the lowest hanging fruit and doesn't require a breaking change¹, in my opinion.

🧬 QA & Testing

I updated our test bed to reflect this, but you can test this out yourself by checking out this branch and running the following command:

npm run debug -- changelogs README.md

Footnotes

1. The reason I don't think it's a breaking change is because we require a title attribute on every page type, so if a Markdown file lacked frontmatter attributes, rdme would almost certainly error out on it. With this change, rdme will skip over it instead and proceed with everything else as normal. ↩