[docs] Add Joy UI theme typography page

[siriwatknp]

Preview: https://deploy-preview-34811--material-ui.netlify.app/joy-ui/customization/theme-typography/

This PR adds documentation about theme typography for Joy UI.

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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[mapache-salvaje]

Looking good overall! Great job as always @siriwatknp.

In addition to my suggestions here, can I request that we rename MoreTypographyLevel.js to either MoreTypographyLevels.js (plural, to be grammatically correct) or NewTypographyLevel.js (singular, since the demo only adds one level)?

[mapache-salvaje]

I think it's worthwhile to be explicit here.

Suggested change

	- The `h*` levels are mostly headlines that follow semantic HTML headings.
	- `h1` through `h6` - header tags that follow the standard semantic HTML headings.

[siriwatknp]

Looking good overall! Great job as always @siriwatknp.

In addition to my suggestions here, can I request that we rename MoreTypographyLevel.js to either MoreTypographyLevels.js (plural, to be grammatically correct) or NewTypographyLevel.js (singular, since the demo only adds one level)?

Updated to NewTypographyLevel, thanks for the suggestion.

[mapache-salvaje]

Looks good! My last open comment here is just tiny nitpicking.

[oliviertassinari]

Let's please use {{"demo": "…", "iframe": true}} the next time for this type of demos. It's a pain to keep in sync in https://codesandbox.io/dashboard/sandboxes/used%20in%20docs?workspace=836800c1-9711-491c-a89b-3ac24cbd8cd8 and it's slower to load than it needs to be for developers.