Add ability to hide inline TOC and use generated TOC

In some imported docs (imported from AMPHTML repo), they have their own "hard-coded" TOC but we want to hide that TOC and use the generated TOC. For these reasons:

1. It muddies the doc to have inline toc and generated. Generated is better because it's fixed to side so the user can scroll the doc and still have the TOC handy.
2. The inline TOC's anchor syntax isn't the same as what's on ampproject.org, so some of the inline TOC links won't work.
3. The inline TOC isn't always maintained. Generated TOC is fresher.

Sample docs:

- https://www.ampproject.org/docs/analytics/amp-managing-user-state
- https://www.ampproject.org/docs/ads/a4a_spec