Display warning of outdated content on technical blog posts

All the material we write will be outdated sooner rather than later, and — at least as per @casperdcl and @0x2b3bfa0 — blog posts aren't something we should maintain to the same extent as documentation.

It would be nice to display a warning of outdated content on technical blog posts when a given amount of days/months passes.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](https://www.gatsbyjs.com/docs/how-to/routing/adding-markdown-pages/#frontmatter-for-metadata-in-markdown-files).

```yaml
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 See https://docs.github.com/es for a similar notice on outdated translations.

> _Frecuentemente publicamos actualizaciones de nuestra documentación. Es posible que la traducción de esta página esté en curso. Para conocer la información más actual, visita la [documentación en inglés](https://docs.github.com/en). Si existe un problema con las traducciones en esta página, por favor [infórmanos](https://github.com/contact?form[subject]=translation%20issue%20on%20docs.github.com&form[comments]=)._

> _We frequently publish updates to our documentation. Translation of this page may be in progress. For the most current information, please visit the [English documentation](https://docs.github.com/en). If there is a problem with the translations on this page, please [inform us](https://github.com/contact?form[subject]=translation%20issue%20on%20docs.github.com&form[comments]=)._

Start by locating how this Gatsby theme renders technical blog posts and reads Markdown frontmatter. Review the proposed expiry date or override behavior, then define the warning's display conditions and verify that posts past the configured threshold show it while current posts do not.

Written by the indexing model from the issue text.