Add documentation for link templates

@apaderno:

Thanks for updating the issue description.

I wrote the documentation page, and @Berdir added it to the menu (see Comment #9). I think this issue is complete, but I want to follow the usual procedure of setting the status to NR (Comment #6) and letting others update the status to RTBC, then Fixed.

@benjifisher I apologize for bumping this issue.
I was going through all these issues to understand which ones were already fixed and which ones I could fix. I moved the link for the "involved" page at the beginning to make it easier for other users to understand which page was "involved."

To me, the information given on that page seems complete.

Hi Everyone!

I just read through the page ( https://www.drupal.org/docs/drupal-apis/entity-api/link-templates → ) for review. I can confirm it's been added to the menu and that the information is adequate in detail. It also seems to conform to the API docs commenting and standards described here: https://www.drupal.org/docs/develop/standards/php/api-documentation-and-comment-standards →

This is actually my first comment on any issue, so I am not certain if that means I can update the status to RTBC, but echoing the other commenters above, everything seems to be in order on this one!

Setting to NW because even though the link templates docs are good (thanks!), they stop just short of telling me the spec for *.link_relation_types.yml files, or giving the context for how to figure out where to look. The two linked ones have URIs that are 404s, and then a description, so I suppose I can get away with just about anything, as long as the top-level key is defined, but it would be nice to know.

The other thing is this one: #2848945: EntityType link templates: using link relation type names as keys is a problem → which hints that maybe we should be careful how we define these, in line with an IANA standard, and it seems like referencing that standard in a relevant way on the docs page would be a good clue.

I'd update it, but I have yet to learn what to say. :-)