fix: convert remaining raw HTML to markdown and add acronym shortcodes

fix: convert remaining raw HTML to markdown and add acronym shortcodes

- myself-as-a-droid: drop external image figure, decode entities
- poker-time: drop external image figure and orphan nbsp
- le-email-queste-sconosciute: replace acronym tags with shortcodes, markdown strikethrough
- xrandr-dual-head: convert pre block to fenced code
- git-setup-own-server-part2: markdown blockquote + drop image figure + fenced code blocks (user will add image later)