Rebuild landing page menu

[AshGDS]

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What / Why

• Adds a new main menu block, to replace the main navigation block
• I created a new block as the backend will work differently now that the nesting is gone and there's headings. And I thought the git diff might look a bit too confusing.
• I haven't removed the main_navigation block from the code as there's some backend related stuff and backend docs I wasn't sure what to do with.
• NB: The backend devs will need to do some work to populate the links properly again.
• Trello card

Screenshots?

Button left-alignment quirk

• In the designs the button is always perfectly left aligned to the govuk-width-container.
• However, as the expanded button styles are different to the collapsed styles, the button has different padding depending on whether it's expanded or collapsed.
• Currently I have given the collapsed button the same padding as the expanded one. However this makes it not look perfectly left aligned when it's collapsed.
• I can make it more aligned when collapsed, but each option has tradeoffs. The first option would cause layout shift,
• See videos below for the two options:

1. Current styles (no layout shift, but worse left alignment when collapsed)

Shift.1.mov

2. Different Padding (which causes layout shift, but better left alignment when collapsed)

Button.Shift.2.mov

3. -15px margin left (no layout shift, better left alignment, but the tab border when expanded won't be left aligned)

Option.3.mov