Design: Add new slot to the Footer

[AlexSkrypnyk]

Consumer sites have use cases where they want to have a more simplified footer that is just a horizontal list of links. The Navigation component already allows to output menu links in a horizontal line, but the Footer does not have a slot where these links can be placed

If the above proposal is accepted, then the following changes would need to be done:

1. Figma to create a variant with a horizontal navigation
2. UIKit to provide an extra slot content_middle5
3. Drupal theme to declare a new region (and also include it into the starter kit)

Please note that #2 and #3 can still happen without #1 to allow consumer sites users to make their own decisions on the menu placement.

[AlexSkrypnyk]

Point 2 implemented in #401

[alanrako]

@fionamorrison23
As discussed please confirm the following:

1. Does the horizontal slot replace vertical columns?
2. Do we also introduce a new variant where the horizontal slot is positioned beneath the acknowledgement of country?

[fionamorrison23]

@alanrako

1. No, the new slot does not replace the columns, it's in addition AFAIK. But there may be cases where the columns are not used.
2. And yes, I do think we need to consider an option for a horizontal slot below the Acknowledgement of Country. But perhaps we need to ask if we need two new horizontal slots.

[alanrako]

@fionamorrison23

Updates made as per description - for approval.

We now have 3 footer variants for approval:

1. Vertical navigation
2. Vertical & horizontal centre
3. Vertical & horizontal bottom

Desktop designs attached.
Figma design here

We also now have 4 descendant mobile menu component variants:

1. Stroke top
2. Stroke bottom
3. Stroke top and bottom
4. Stroke none

Figma design here