Display warning of outdated content on technical blog posts

[0x2b3bfa0]

All the material we write will be outdated sooner rather than later, and — at least as per @casperdcl and @0x2b3bfa0 — blog posts aren't something we should maintain to the same extent as documentation.

It would be nice to display a warning of outdated content on technical blog posts when a given amount of days/months passes. Additionally, it would be nice to override/configure this through the post frontmatter.

best-before: 2022-08-01

Note
Playful example, not suggesting best-before as the best name; also not sure if it's better to use relative (in 6 months) or “absolute” (2022-12-27) time.

Examples

Can't find any off the top of my head, but I have seen a few in the last fortnight. See https://docs.github.com/es for a similar notice on outdated translations.

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[casperdcl]

/CC @iterative/docs

[jorgeorpinel]

I feel like it's hard to determine whether something is outdated based on a pre-defined length of time. Some features are stable and will apply almost the same years later. Some things from release posts for example, in contrast, may be outdated a few months later. But you don't know until it happens.

Blog posts already have a date in any case so readers can decide whether it may be outdated or not.

p.s. we can always add these warnings manually to specific posts if needed.