feat: use next/image for MDX images

[saidstack]

Fixes #4382

What changed

Replaced the plain <img> in the MDX Image component with next/image,
so markdown images and raw <img> tags now get lazy-loading, async
decoding, and image optimization.

Changes

• Added import NextImage from 'next/image' in src/components/MDX/MDXComponents.tsx
• Updated the MDX Image component to render next/image instead of <img>

Open question

Markdown images usually don't specify width/height. I used a 700×400
fallback for dimension-less images. Is that acceptable, or is there a
preferred approach (e.g. fill + a sized container, or reading intrinsic
dimensions)?

That Fixes #4382 is what the template was asking for — it auto-links (and eventually auto-closes) the issue when the PR merges.

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

Five Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/404	127.56 KB (🟡 +1 B)	242.71 KB
/500	127.57 KB (🟡 +1 B)	242.72 KB
/[[...markdownPath]]	130.01 KB (🟡 +1 B)	245.16 KB
/errors	127.81 KB (🟡 +1 B)	242.96 KB
/errors/[errorCode]	127.79 KB (🟡 +1 B)	242.94 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.