Add 'id's to headings in the service manual

[mcheung-nhs]

This would make it easier for it to be referenced and to link to specific sections of the service manual
E.g. on https://service-manual.nhs.uk/design-system/patterns/ask-users-for-their-nhs-number
there is a section further down the page with the heading "How to build the NHS number pattern"
If this had an id on it, such as
<h3 id="how-to-build-the-nhs-number-pattern">How to build the NHS number pattern</h3>
this would make it easier for users to reference this section and jump to it using
https://service-manual.nhs.uk/design-system/patterns/ask-users-for-their-nhs-number#how-to-build-the-nhs-number-pattern
The use of ids has been used in other pages and headings but not on all the headings.