docs: add more documentation on how to do theme overrides

[matyasf]

Content mostly recovered from the deleted parts in the v9 version, but I also tried to make the examples more clear

[github-actions]

PR Preview Action v1.4.8
🚀 Deployed preview to https://instructure.github.io/instructure-ui/pr-preview/pr-1800/
on branch gh-pages at 2024-12-03 11:08 UTC

[balzss]

looks good, I'd add an extra section about overriding "nested" components. e.g. if you want to override the borderRadius for DateInput2 you cannot do it directly and you have to do it like this:

<InstUISettingsProvider
  theme={{
    componentOverrides: {
      TextInput: {
        borderRadius: '50%'
      }
    }
  }}
    >
    <DateInput2 />
</InstUISettingsProvider>