update React form component urls

[ktravers]

I noticed that the component links in the React Forms doc were broken while reviewing https://github.com/github/github/pull/362188.

The url scheme seems to have changed since these docs were last updated. Looks like it now follows the pattern https://primer.style/components/:component-name/ for interface guidelines and https://primer.style/components/:component-name/react/latest for React implementation.

Questions for reviewers:

• Are there any url helpers available? Hard-coding the urls seems brittle and could break again if/when the url scheme changes.
• I've seen "dead link" linters in other repos that fail if any urls in the PR return a >4XX response (example). Does something like that seem useful for this repo?

[TylerJDev]

Are there any url helpers available? Hard-coding the urls seems brittle and could break again if/when the url scheme changes.

I agree. I believe there was discussion around this recently for our new docs. I'm forgetting what came of it, but @joshblack might know!

Thanks for the PR too ✨