docs(Theme): update Theme docs with new guidance/usage

[dakahn]

partially closes #11293

Updates the /code page for Themes with new usage/guidance clarity based on some support work we've done

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Name	Status	Preview	Updated
carbondesignsystem	✅ Ready (Inspect)	Visit Preview	Sep 26, 2022 at 0:49AM (UTC)

[tay1orjones]

I've totally overhauled this one with a bunch of theme related changes. Please re-review and let me know what you think.

[tw15egan]

Changes look good to me!

Since you say

If you're using @carbon/react there's no reason to install the themes package separately. See our Carbon React guide to start

does that mean in the examples, you can just do @use '@carbon/themes and only have @carbon/react installed?

[tay1orjones]

@tw15egan No! Good distinction. In this case it means that instead of having @carbon/themes installed and doing:

@use '@carbon/themes/scss/theme';

You can instead access this stuff from @carbon/react

@use '@carbon/react/scss/theme';

This is limited to just the styles as well. If someone's wanting to get the exports for usage in javascript, they'll need to install the @carbon/themes package.

I'll update this verbiage to say "you likely don't need to install the themes package" instead of "there's no reason to install the themes package"