USF-1510: Cart Summary has a Footer Slot

[anthoula]

• add footer slot example to the boilerplate

Please always provide the GitHub issue(s) your PR is for, as well as test URLs where your change can be observed (before and after):

Pull Request

• Dropin SDK - 0.31.0-alpha005
• Cart Dropin - 0.7.0-alpha010
• Documentation

Test URLs:

• Before: https://develop--aem-boilerplate-commerce--hlxsites.aem.live/
• After: https://USF-1510--aem-boilerplate-commerce--hlxsites.aem.live/

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Commits

* [0ad2cd6](https://github.com/hlxsites/aem-boilerplate-commerce/commit/0ad2cd6017b8eef495950f6f98b97df77abc51e6) :arrows_counterclockwise: (latest) * [a1ab941](https://github.com/hlxsites/aem-boilerplate-commerce/commit/a1ab9415c4687ae96f3db9a2b30f9a6f4b119dcb) :white_check_mark: * [4ce20d3](https://github.com/hlxsites/aem-boilerplate-commerce/commit/4ce20d3058917b76a64d9f78db3c8c2a66c08ff8) :white_check_mark: * [26f2761](https://github.com/hlxsites/aem-boilerplate-commerce/commit/26f276149039f87bc62c5915145b4897754170ca) :white_check_mark: * [43d2f6b](https://github.com/hlxsites/aem-boilerplate-commerce/commit/43d2f6bb14e9eddf3c0d132783ae56c7485f84bc) :white_check_mark: * [c0520ff](https://github.com/hlxsites/aem-boilerplate-commerce/commit/c0520ff3e6fa7a823761cbdd2e87d5c97da9c366) :white_check_mark:

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