Update to match the new /docs/README.md Triggers Overview from PR 972
Update to match the new /docs/README.md Triggers Overview from PR 972

Updates the top-level project README.md to link to the new Triggers Overview from https://github.com/tektoncd/triggers/pull/972 and remove overlapping content. Also updates the language for clarity and flow.
tektoncd/triggers,tektoncd/triggers