[SBTW-2122] Add ToC into sample README.md#1301

HoonBaek merged 2 commits intomainsendbird/sendbird-uikit-react:mainfrom

docs/sbtw-2122/toc-and-quick-linkssendbird/sendbird-uikit-react:docs/sbtw-2122/toc-and-quick-linksCopy head branch name to clipboard