Define a link to discover API

In order to make it easier for users to use CKAN api's, it might be nice if a standard link would be defined, that can appear on portal homepages.

2 examples:

```html

```

Such a link could allow an application to ask a user for a CKAN url, and that application could automatically discover where the related API is.

It also allows people to add the same link on other, related open data pages so that the CKAN api root can be discovered from a wide variety of urls.

My specific use-case is a google sheets add-on. I want users to be able to add their CKAN instances, but asking them what the API root is might be quite difficult for the target audience.

If they give me (some) url, and this url is either: 1) The API root or 2) contains a link to the API root, I can reduce the friction quite a bit in adding new instances.

Review the proposed HTML link forms and the two API-discovery cases described in the issue. Done would require a decided standard and agreed implementation scope, but this payload identifies no files, tests, or entry points.

Written by the indexing model from the issue text.