Fix TypeScript errors in "Add content to your site" code snippets

[ArmandPhilippot]

Description (required)

Follow-up of #14089. TL;DR: prevent red squiggles in our code snippets to improve UX.

Fixes code snippets in "Add content to your site":

• images.mdx:
  • TypeScript complains without an explicit alt on <Image />
  • getElementById can return null
• content-collections.mdx:
  • missing imports
  • consistent quotes (single/double in the same code snippet), see Fix TypeScript errors in "Configuration" & "Routing and navigation" code snippets #14089 for the reasoning about formatting
  • getEntry()/getLiveEntry() may return undefined
  • a schema above defined pubDate not published
  • not fix for undefined properties (ie. data.*) since they depends on a schema not shown here
  • Astro.params.slug is incompatible with getLiveEntry() without an if statement
  • Astro.params.id can be undefined
  • TypeScript complains about post.frontmatter.title without post: any (no type for import.meta.glob)

To help review and explain my changes I made a commit per code snippet (with a few exceptions where the changes were the same), with a short explanation of why in the commit message.

Related issues & labels (optional)

• Suggested label: code snippet update

[astrobot-houston]

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Tracked Files

File	Note
en/guides/content-collections.mdx	Source changed, localizations will be marked as outdated.
en/guides/images.mdx	Source changed, localizations will be marked as outdated.
en/guides/markdown-content.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[github-actions]

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[yanthomasdev]

LGTM!