Add charset note md/mdx pages

src/content/docs/en/basics/layouts.mdx

@@ -234,6 +234,12 @@ const { title, fancyJsHelper } = Astro.props;
 
 <ReadMore>Learn more about Astro’s Markdown and MDX support in our [Markdown guide](/en/guides/markdown-content/).</ReadMore>
 
+:::note
+
+When a layout is not used, Astro will automatically add a `<meta charset="utf-8">` tag to your page to allow easier authoring of non-ASCII content. If you are using a layout for non-ASCII content, you should include the `<meta charset="utf-8">` tag in your layout as Astro will no longer add it automatically.
+
+:::
+
 ## Nesting Layouts
 
 Layout components do not need to contain an entire page worth of HTML. You can break your layouts into smaller components, and combine layout components to create even more flexible, page templates. This pattern is useful when you want to share some code across multiple layouts.

src/content/docs/en/guides/upgrade-to/v5.mdx

@@ -752,6 +752,22 @@ Code blocks in `.mdoc` files and Astro's built-in `<Code />` component do not us
 
 <ReadMore>Read more about [syntax highlighting in Astro](/en/guides/markdown-content/#syntax-highlighting).</ReadMore>
 
+### Changed: Automatic `charset=utf-8` behavior for Markdown and MDX pages
+
+<SourcePR number="12231" title="Unset charset=utf-8 content-type for md/mdx pages"/>
+
+In Astro 4.0, Markdown and MDX pages (located in `src/pages/`) automatically responded with `charset=utf-8` in the `Content-Type` header, which allowed rendering non-ASCII characters in your pages.
+
+Astro 5.0 updates the behaviour to add the `<meta charset="utf-8">` tag instead, and only for pages that do not use Astro's special `layout` frontmatter property. Similarly for MDX pages, Astro will only add the tag if the MDX content does not import a wrapping `Layout` component.
+
+If your Markdown or MDX pages use the `layout` frontmatter property, or if the MDX page content imports a wrapping `Layout` component, then the HTML encoding will be handled by the designated layout component instead, and the `<meta charset="utf-8">` tag will not be added to your page by default.
+
+#### What should I do?
+
+If you require `charset=utf-8` to render your page correctly, make sure that your layout components contain the `<meta charset="utf-8">` tag. You may need to add this if you have not already done so.
+
+<ReadMore>Read more about [Markdown layouts](/en/basics/layouts/#markdown-layouts).</ReadMore>
+
 ### Changed: Astro-specific metadata attached in remark and rehype plugins
 
 <SourcePR number="11861" title="Clean up Astro metadata in vfile.data"/>