Update information about react-tokens to docs

[nicolethoen]

react-tokens map to most or all CSS variables in patternfly, and there is no way for a consumer to look them up if they want to use them. This will become ever more important as we used versioned prefixes in our CSS, since the CSS selectors and variables will change every major release, but our react-tokens wont.

our CSS variables tables in component docs and Global CSS Variables page should include information about the corresponding react-tokens.

[nicolethoen]

May need to wait to start this until we have more clarity on how the theming work will impact react-tokens