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

[hegemonic]

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.