Update @fluentui/react code examples in styles section of website

[ecraig12345]

The website styles section currently does not have good information about how we actually recommend that people do styling with Fluent UI React. Some individual things that need to be fixed (non-exhaustive list):

1. Replace @uifabric/fluent-theme references with @uifabric/styling where possible.

• A few constants are not currently exported from the styling package. We should probably move them there so people can entirely stop referencing @uifabric/fluent-theme.

2. Add examples of how we actually want people to do styling--getting values such as colors and font sizes from the theme, not constants.
3. Some pages (example) have zero implementation info because they're just meant for reference and not available as variables. We should probably add a skeleton implementation section noting this.
4. The Layout page needs a note about Stack vs grid--see Add note about grid vs Stack to layout styles page on website #13380

[ecraig12345]

Improved somewhat in #14545 but still needs work.

[msft-fluent-ui-bot]

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