Add redirects for /react sub-site

[joshblack]

First part of https://github.com/github/primer/issues/3334

From	To
/react	/
/react/getting-started	/guides/react
/react/theming	/guides/react/theming
/react/theme-reference	/guides/react/theme-reference
/react/linting	/guides/react/linting
/react/system-props	/guides/react/system-props
/react/core-concepts	/guides/react/core-concepts
/react/philosophy	/guides/react/philosophy
/react/overriding-styles	/guides/react/overriding-styles
/react/status	/guides/status
/react/hooks/useSafeTimeout	/guides/react/hooks/use-safe-timeout

[camertron]

LGTM! Would recommend testing this in staging first, but otherwise 🚀

[joshblack]

Testing the redirect links above on the staging site seem to work as expected! Tested a couple of the other redirects, as well, to verify they still worked. Will merge this in and monitor things 🤞