docs: fix broken links in contributing guide, devtools README, and ex…

[shankar-vanapalli]

This PR fixes 5 broken internal links found across the repository's contributing guides, component documentation, and examples.

Specifically, it updates:

• contributing/core/developing.md: Fixed the link to the dev overlay README (moved from react-dev-overlay/ to next-devtools/).
• packages/next/src/next-devtools/README.md: Fixed the link to the dark theme styling file (moved/renamed from ui/styles/dark-theme.tsx to dev-overlay/styles/dark-theme.css).
• examples/cms-graphcms/README.md: Updated the preview mode documentation link from the old advanced-features path to the current Next.js documentation URL.
• examples/cms-umbraco-heartcore/README.md: Updated two occurrences of the preview mode documentation link from the old advanced-features path to the current Next.js documentation URL.
• examples/with-rematch/README.md: Added the missing https:// scheme to the api.github.com/users markdown link so it correctly routes to an external URL instead of resolving as a relative file path.