Add ability to disable preview links per collection

**Is your feature request related to a problem? Please describe.**
Currently, there doesn't seem to be any way to disable preview links per collection.
We have `show_preview_links` which will disable preview links globally.

**Describe the solution you'd like**
Either something like:
```yml
collections:
- name: 'authors'
# …
preview_path: false
```

or

```yml
collections:
- name: 'authors'
# …
preview_links: false # to avoid confusion with global 'show_preview_links' option
```

Or any other variant really.
Perhaps global setting would have precedence local setting.

**Additional context**
It would be useful to have something like this for collections that don't actually have a page. For instance, I have an authors collection, but there's no authors page. Authors are related to blog posts. And blog posts need a preview link. Authors don't.

Start by tracing how the global show_preview_links setting and collection configuration control preview-link rendering. Compare the requested per-collection options and determine the precedence between global and local settings. Done means collections without pages can suppress preview links while collections that need them retain them, with coverage for both settings.

Written by the indexing model from the issue text.