Add "table of contents" for the landing pages with lots of content? #2110

Pages like the Datastore landing page (https://googlecloudplatform.github.io/google-cloud-node/#/docs/google-cloud/0.49.0/datastore) have a ton of useful content, but there's no quick way to navigate to each section (e.g., "I forgot how to do transactional stuff... I just want a reminder about those transaction things").

Can we add a TOC-type box (maybe like the one that shows the available methods) for these pages? For example (I could just click "Transactions" in the box)

(Though we may want to change some of the headings now that they're front-and-center).