TWE-62: Add PromoBlock to ServiceAreaPage (with link optional)

[bmispelon]

Link to Ticket

Description of Changes Made

This PR adds a PromoBlock to the ServiceAreaPage page type. Unlike the core PromoBlock, this custom one doesn't require the editor to provide a link_text or a link_button. Note however that if a link_text is provided, then link_button is required.

How to Test

1. Create (or edit) a ServiceAreaPage
2. In the body field, add a "Promo" block
3. Observe that neither "Link text" nor "Link Button" are marked as required
4. Publish the page and view it
5. See that the promo block is visible, but it has no button

Screenshots

The admin side:

The resulting page

MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Tests and linting passes.

Unit tests

• Added
• Not required

Documentation

• Updated build docs
• Updated editor guidelines (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• ⚠️ Updated field spec (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only) This needs to be done once PR has been approved
• Not required

Browser testing

• I have tested in the following browsers and environments (edit the list as required)
  • Latest version of Chrome on mac
  • Latest version of Firefox on mac
  • Latest version of Safari on mac
  • Safari on last two versions of iOS
  • Chrome on last two versions of Android
• Not required

Data protection

• Not relevant
• This adds new sources of PII and documents it and modifies Birdbath processors accordingly

Light and dark mode

• I have tested the changes in both light and dark mode
• The change is not relevant to dark and light mode

Accessibility

• Automated WCAG 2.1 tests pass
• HTML validation passes
• Manual WCAG 2.1 tests completed
• I have tested in a screen reader
• I have tested in high-contrast mode
• Any animations removed for prefers-reduced-motion
• Not required

Sustainability

• Images are optimised and lazy-loading used where appropriate
• SVGs have been optimised
• Performance and transfer of data considered
• If JavaScript is needed alternatives have been considered
• Not required

Pattern library

• The pattern library component for this template displays correctly, and does not break parent templates
• The styleguide is updated if relevant
• Changes are not relevant the pattern library

[SharmaineLim]

LGTM! :)