chore: Fix WordPress migration artifacts in blog content

[pReya]

Description

Clean up leftover WordPress artifacts from the blog content migration. No content changes — purely markup/URL cleanup.

WordPress shortcodes (2 files)

• Converted 4 [embed] shortcodes to {{ youtube_player(video_id="...") }} Zola shortcodes
• Converted 1 [caption] shortcode to a proper / HTML element

Unused CSS classes (108 files)

• Removed 211 wp-image-XXX classes from tags — these are WordPress post-attachment IDs with no corresponding CSS rules in the Zola theme

Stray <br> tags (17 files)

• Removed 51 standalone and trailing
  tags that were carried over from WordPress's visual editor output

Absolute wp-content URLs (27 files)

• Converted 35 http://matrix.org/blog/wp-content/ and //matrix.org/blog/wp-content/ URLs to root-relative /blog/wp-content/ paths — the images already exist in static/blog/wp-content/

DISCLAIMER: These changes were created with the help of an LLM (Claude Sonnet). However I still reviewed them manually and I am creating this PR manually.

I might add other/more of these cleanup PRs – there's a lot more to do, but I don't want these PRs to grow too big so they can't be reviewed anymore.

[MTRNord]

Some general nit: the other classes are also non existent. Why didnt they get removed?

also how about using the figure shortcode instead like in https://github.com/matrix-org/matrix.org/blob/main/CONTENT.md#adding-pictures-in-your-post ?

also wondering about how feasible it would be to use the zola relative link syntax instead https://www.getzola.org/documentation/content/linking/#internal-links or at least regular relative links instead of having https://matrix.org hardcoded as this technially also breaks the cloudflare previews. Though not sure how relevant that in reality is for the old posts.