Add identifierItems for TaskGroupRenderSection

[Kyle-Ye]

Bug/issue #, if applicable:

Close #480

Summary

Update identifiers to identifierItems in TaskGroupRenderSection

## Overview

For more information see the [specification](https://example.com).

## See Also
- [Name 1](https://example.com)

- [Name 2](https://example.com)

- [Name 3](https://example3.com)

- [this great link ![random image](https://picsum.photos/200)](https://example3.com)

Before the PR

JSON output:

"seeAlsoSections": [
    {
        "title": "Related Documentation",
	    "identifiers": [
	      "https://example.com",
	      "https://example.com",
	      "https://example3.com",
	      "https://example3.com"
	    ]
    }
]

swift-docc-render:

After the PR

JSON output:

"seeAlsoSections": [
  {
    "title": "Related Documentation",
    "identifiers": [
      "https://example.com",
      "https://example.com",
      "https://example3.com",
      "https://example3.com"
    ],
    "identifierItems": [
      {
        "identifier": "https://example.com"
      },
      {
        "overrideTitle": "Name 2",
        "overridingTitleInlineContent": [
          {
            "type": "text",
            "text": "Name 2"
          }
        ],
        "identifier": "https://example.com"
      },
      {
        "identifier": "https://example3.com"
      },
      {
        "overrideTitle": "this great link ",
        "overridingTitleInlineContent": [
          {
            "type": "text",
            "text": "this great link "
          },
          {
            "type": "image",
            "identifier": "https://picsum.photos/200"
          }
        ],
        "identifier": "https://example3.com"
      }
    ]
  }
]

swift-docc-render:

Dependencies

Swift-DocC: #376 (Merged)
Swift-DocC-Render: swiftlang/swift-docc-render#691

Testing

[TODO]

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[franklinsch]

This approach makes sense to me. It would be good to align with the approach we've taken with the overridingTitleInlineContent parameter of RenderInlineContent.reference so that we support formatting in the title for extra flexibility.

[Kyle-Ye]

This approach makes sense to me. It would be good to align with the approach we've taken with the overridingTitleInlineContent parameter of RenderInlineContent.reference so that we support formatting in the title for extra flexibility.

Update the swift-docc side implementation with your suggestion.

And the swift-docc-render PR is half-ready.

[mportiz08]

I have a minor concern about the proposed Render JSON changes here. I like that the new identifierItems section was added as a way to maintain backwards compatibility for the new feature as opposed to modifying the existing identifiers array structure.

However, it seems that the new array will always be presented, even in the case where it isn't needed for the more common scenario where none of the links have custom titles. I'm slightly worried that this will add to the size of large libraries of content since the object with the duplicated identifier will be added for each see-also item.

Is there a way to make this an optional field that only gets included in Render JSON when it's needed for custom titles?