Tech docs converts punctuation to HTML entities in IDs

[hannalaakso]

What

If a heading includes for instance an apostrophe it gets converted to its entity reference number in the resulting id.

So for instance

## Turn on 'compatibility mode'

results in the URL https://frontend.design-system.service.gov.uk/compatibility-mode/#39-compatibility-mode-39

and

## Find out which version you're using

becomes https://frontend.design-system.service.gov.uk/updating-with-npm/#find-out-which-version-you-39-re-using

Why fix this

The URLs end up looking pretty weird, this might or might not be an issue for users.