Fix the legacy posts imported in Zola

Todo once #1511 #1512 #1513 #1514 #1515 #1516 #1517 #1518 and #1519 have been merged:

- [ ] Replace all `` blocks in the markdown by actual code blocks
- [ ] Replace all `

` blocks by actual code blocks

- [ ] Replace all `` links by `[]()` links

- [ ] Download "legacy" images, add them next to the post markdown, and replace `` tags by `![]()` 

- [ ] Replace all `
`/`
` `
` by `- `

- [x] Remove ` `

- [x] Replace `&` with `&`

- [x] Replace `` by X*`#` (from h1 to h5)

- [ ] Remove `
` tags

- [ ] Replace `[embed]` with https://regex101.com/r/nQOzcm/1

- [ ] Replace `[caption]`s with *something*

- [ ] Replace `` and `` tags with `**this is bold**`

- [ ] Replace `` and `` tags with `*this is italics*`

- [ ] Remove `
` (let's check none is super important first)

- [ ] Remove `
` `
` `
` tags

- [ ] Remove non-standard tags (``??)

- [ ] Make sure we have no h1 or # level-one headings within posts

- [ ] Crawl media and make sure none is broken

- [ ] When youtube players are embedded, at least make sure `loading="lazy"` exists

- [x] Replace `
` with `> `

- [ ] Replace `
` with `---`

- [ ] Remove `
` or replace with `\` at the end of the line

- [ ] List all markdown files with `` tags and check what they do?

- [x] Replace `<del>` with `~~`? Depending on the markdown flavour we support

- [ ] List all markdown files with `<div>` tags and check what they do?

- [ ] Update the YouTube embeds so they lazy load and have a decent size by default?

- [x] Remove empty `class="anchor"` links

- [ ] Add `<!-- more -->` cutoff points for summary generation (to make the blog article listings easier to skim)

Start by reviewing the legacy post markdown imported in Zola after issues #1511 through #1519 are merged. Work through the unchecked conversion, media, embed, tag, and summary tasks, and consider the issue done when the checklist is complete and legacy posts no longer contain the listed HTML problems.

Written by the indexing model from the issue text.