Markdown headings don't have IDs so can't be linked to

[sebpearce]

We need to decide whether to:

1. generate dynamic IDs in snake-case based on the heading text, or
2. use fixed IDs that are unrelated to the text

[dougmacknz]

I can take this one on. I just spoke to @didoesdigital about the options above, we reckon dynamic IDs would probably be the best, in case things change quite heavily and then you've got a URL that takes you somewhere that doesn't make sense anymore.