Link to any types defined in, and exported by, index.ts

From @dansaadati in #77:

> [S]ome libraries (especially ones that aren't generated through the GAPIC template, like `nodejs-dns`) can sometimes define and export a type from the `index.ts` file, which means we'll see the type declared directly in the `index.d.ts` file. Example from `nodejs-dns`:
>
> ```
> export interface CreateZoneRequest {
> dnsName: string;
> ...
> }
> ```
>
> We have a couple of options here:
>
> 1. Leave behavior as is. This means the items can be linked to, but won't directly appear in the TOC.
> 2. Expand this regex a bit to capture the type, and then include it in the TOC based on the type (e.g. interfaces will go under interface).
>
> I think we can start with option 1, and then get a sense of whether we should include those items in the TOC. The downside is, when they are clicked on, the left nav will not be visible (e.g. https://cloud.google.com/nodejs/docs/reference/dns/latest/dns/createzonerequest). In any case, this is more of a follow-up to this PR!

This feature request is to implement option 2.