Update the hero and add a description to the service home page

[markjefferson-gov]

What

Update the service start page hero to:

• Improve the hero header (H1)
• add a hero description with a link to a more detailed description (this will link to the user guide to begin with)
• change the search field label

As per this mockup:

Copy changes:

• H1: Discover data from across the Ministry of Justice
• Description: Find MoJ data shows you what data exists and how to access it. Learn more
• Search label: Search for data

Why

These changes will help new users quickly understand the service and decide what to do next.

Done when

• Copy changes are drafted by @markjefferson-gov and approved by the team

• Copy changes have been implemented in production

[markjefferson-gov]

I've shared a proposal for this with Sean and Yvan. Waiting for feedback.

Part of the proposed changes in: https://miro.com/app/board/uXjVKuHRwjI=/?share_link_id=797463948769

[markjefferson-gov]

Discussed in standup. Copy and ticket updated and moved out of blocked.

[LavMatt]

going to do this alongside #637