Improve readability

[Malvoz]

Primarily:

• Remove bold text
  (per Simon's feedback in Review of MapML-Proposal explainer #17, I agree with that, so removed all bold text for now.):
  
  

  (Note: I found the bold formatting makes this harder for me to read.

• Rename headings that are hard to digest
• Add syntax highlighting to blocks of code
• Add missing backticks (I think 1 instance of <area> and <img> are currently being rendered instead)

Additionally, off-topic:

• Change https://maps4html.github.io/ URLs to https://maps4html.org/
• Change the link for HTML link relations from MDN to the WHATWG definition

[prushforth]

Thanks, Malvoz and Simon! Let's continue the improvement process.