Upgrade deps (Gatsby 5.11) and minor improvements

[fturmel]

Upgraded all the dependencies I could. Noteworthy:

• Gatsby was upgraded from 5.7 to 5.11
  • Version 5.12 was a no go, unfortunately. They introduced hosting adapters which supersedes gatsby-plugin-netlify, but it does not look ready at all (lots of regressions and it doesn't work with Gatsby esm configs). Tracking Gatsby Adapters causes regressions & issues with headers & redirects for NETLIFY builds on >=5.12.0 gatsbyjs/gatsby#38542 and Documentation is wrong for initialising the plugin. gatsbyjs/gatsby#38606
• Prettier v3 .format() is async now, so I refactored the tags-transforms script accordingly
• Can't upgrade to remark-gfm 4 at the moment, I assume there is work to do in gatsby-plugin-mdx still. I saw some babel-loader errors when trying to parse markdown files.

And the minor improvements:

• Added links to guide card images in /guides
• Added a missing React key in the showcase form

[netlify]

✅ Deploy Preview for codingtrain ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	f205889
🔍 Latest deploy log	https://app.netlify.com/sites/codingtrain/deploys/6533f2e22588b10007aeaf96
😎 Deploy Preview	https://deploy-preview-1243--codingtrain.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[shiffman]

Wow, thank you @fturmel! @runemadsen @fdoflorenzano I've looked this through and am going to merge, but wanted to tag you in just as an FYI.

[fturmel]

You're welcome!