[icons][docs] Update README and docs page

[danilo-leal]

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

Aside from detailing/refining the writing a bit more, the primary new information this PR adds/clarifies is the fact that the Icons package only covers Material Icons and not Material Symbols. It feels like relevant information for the future!

https://deploy-preview-41938--material-ui.netlify.app/material-ui/material-icons/

[mui-bot]

Netlify deploy preview

• docs/data/material/components/material-icons/material-icons.md

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[danilo-leal]

Quick bump on this one — quality-of-life type of update! 👍

[danilo-leal]

@samuelsycamore quick bump on this one to get it out the door! 🚢