Add docs for `Container`

[huong-li-nguyen]

Description

I've considered the old PR on this, but made some changes:

• Created an initial overview page for components (we had several places where it became clear that we needed a landing page for components as otherwise it was referencing the Card & Button docs)
• Changed the example for container
• Updated docs on it

Will be merged after https://github.com/mckinsey/vizro/pull/254

Screenshot

The overview page can definitely be improved further, but I wouldn't do that in this PR. So currently it's just a basic version.

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[huong-li-nguyen]

This makes a lot of sense! Let me try to rephrase some of this in a separate PR.

I've added the quick changes already. The rest I'm currently pushing to a separate PR. So there is no time rush of getting these in right now, but if we manage then that's great! 👍