feature: add OutdatedBanner component for outdated articles

[snappygrav]

Description

Adds an OutdatedBanner component that can be displayed on top of blog articles to warn readers that the content may no longer be valid.

Closes #111

Changes

• src/schemas/posts.ts — added optional outdated boolean field to zPost schema
• src/components/outdated-banner.astro — new component with i18n support
• src/pages/fr/blog/articles/[id]/index.astro — conditionally renders the banner when outdated: true
• src/i18n/fr/blog.json — added blog.article.outdated translation keys
• src/i18n/en/blog.json — added blog.article.outdated translation keys

Usage

First of all, you need to locate into src/content/posts/

Then, look for an article that you want to modify the state to "Outdated", for example: src/content/posts/start-ui/en.md

To mark an article as outdated, add outdated: true to its frontmatter:

---
title: 'Start UI'
slug: 'start-ui-opinionated-ui-starter'
date: 2021-11-01
[other information about the article]
outdated: true
---

Screenshots

	Without banner	With banner
🇫🇷
🇬🇧

Notes

• Since articles are split into separate language files (fr.mdx / en.mdx), the outdated flag needs to be set manually in each file independently.

[vercel]

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