[docs] Add examples for adding and removing Typography variants

[mnajdova]

Closes #22257

[mnajdova]

I am hiding this example, as adding the ts module augmentation on the example is being applied on all docs. The instructions are added as code snippets above.

[eps1lon]

I really wish we could isolate the demos regarding module augmentation. We have a couple of places where we use it or should use it. But since these are applied to all files it's quite problematic.

At least so that the type-checking test suite verifies that the module augmentation is working as expected.

[mnajdova]

Agree, this is problematic.. This is the second time I am hitting this issue so far :\

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against ac066f5

[mbrookes]

I think the customization example should go in the typography customization section, and referenced from the component page, not the other way around.

updated

[mnajdova]

I think the customization example should go in the typography customization section, and referenced from the component page, not the other way around.

👍 I swapped the content of the two pages.

[mbrookes]

👍🏼