[docs] Fix alpha usage

[siriwatknp]

alpha() will throw an error if the input is a CSS variable. The usage is incorrect and the reason that the code does not throw error is because the documentation pages are not using CSS variables yet.

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

[mui-bot]

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[alexfauquette]

Would it work with CSS Variable after this PR?

I remembered that at the beginning of CSS vars provider, we had to do stuff like `rgba(${theme.vars.palette.primary.mainChannel} / 0.02)`

[siriwatknp]

Would it work with CSS Variable after this PR?

Yes, it will work.

I remembered that at the beginning of CSS vars provider, we had to do stuff like `rgba(${theme.vars.palette.primary.mainChannel} / 0.02)`

I see. It is different use case, in my change, the color like grey[50] does not change between mode but in your example, the rgba(…) is required because primary.mainChannel is different between modes.

We can replace the usage of alpha() with color-mix() in a separate PR to improve the performance (no runtime-JS) but it's not in scope of this PR.