Improve Table of Contents (from markdown) generation

[jeroenheijmans]

The /resources page has a nice Table of Contents (ToC), generated from the relevant markdown files. Nice!

There are some relevant improvements to make though:

• Prevent generating a ToC when not needed (on /devlog pages there's no need)
• Add emoji or small icons to the headings and ToC entries, so it's easier to navigate visually
• Refactor the marked.ts file to be a bit more readable