Migrate from `markdown-it` to `remark`

[hirasso]

markdown-it plugins with possible remark counterparts:

• markdown-it-video ➡️ remark-oembed
• markdown-it-anchor ➡️ anchor-links
• markdown-it-attrs ➡️ remark-attr
• markdown-it-shiki ➡️ remark-expressive-code