Show author and original date on posts and guides

[ferki]

Quite often the content of posts and guides end up being "fire-and-forget" documents, thus rarely, if ever, getting updated as things shift in the ecosystem.

On one hand, core information and/or faster moving targets (like feature flag documentation recently) are being moved from the website into Rex core.

On the other hand, I'd like to indicate the "freshness" of these documents by showing the original date when they were written (or published). On top of that, showing the original author could also help identifying the contact person in case of further questions regarding the topic covered.