[website] Add product core page

[siriwatknp]

waiting for #27666

Preview: https://deploy-preview-27952--material-ui.netlify.app/branding/core/

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

[siriwatknp]

@oliviertassinari

1. fixed
2. agree, updated.
3. agree, updated.
4. nice catch, fixed.
5. good point, change to responsive instead @danilo-leal what do you think about it?
6. changed to h2, h3 instead but for which one should be h2, I am not sure.
7. fixed
8. @danilo-leal any component you want to replace the DatePicker? or should we go with DatePicker for now?

[danilo-leal]

@siriwatknp @oliviertassinari

Just pushed a bunch of changes. Regarding bullet nº8, I've removed the date picker and added/tweaked some of the components there. I focused on interactivity instead of a simple display of components, so most of them you can interact and play a bit.

Great work, Jun, looking awesome! The interactive responsive slider on the styling section is just too good, very nice touch!

[oliviertassinari]

changed to h2, h3 instead but for which one should be h2, I am not sure.

As far as I know, headers are supposed to be continuous h3 + h2 would break this rule. h2 + h3 sounds OK.