[Docs] Add documentation and examples for `Figure`

[huong-li-nguyen]

Description

• Add How-to-guide for Figure
• Add How-to-guide for custom Figure
• Update features examples dashboard with Figure and custom Figure
• Tidy API reference

https://vizro--524.org.readthedocs.build/en/524/pages/user-guides/figure/ https://vizro--524.org.readthedocs.build/en/524/pages/user-guides/custom-figures/

More cross-links to the KPI card functions can be found in this PR: https://github.com/mckinsey/vizro/pull/529

Screenshot

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[antonymilne]

FYI @stichbury @huong-li-nguyen I pushed ea5cf95 so that the linkchecker ignores links to our built RTD - otherwise it's impossible to add new pages to our docs since it will always fail on CI like you found here.

I've also pushed https://github.com/mckinsey/vizro/pull/535 which will make our checks stricter so that it's not possible to link to a page that doesn't exist in our docs. This means that there's no need to check the built RTD docs any more.

[huong-li-nguyen]

@antonymilne @stichbury - could the two of you take a final look? Thank you! 🙏