docs: Add related key to docs.json

[jmooring]

hugo config --format json | grep related -A 20 yields:

"related": {
    "indices": [
      {
        "name": "keywords",
        "type": "basic",
        "weight": 100
      },
      {
        "name": "date",
        "type": "basic",
        "weight": 10
      },
      {
        "name": "tags",
        "weight": 80
      }
    ],
    "threshold": 80
  },

It would be convenient if that were in docs.json, but we'd have to note in the docs (special case) that this bit is only present if tags is one of the taxonomies:

      {
        "name": "tags",
        "weight": 80
      }

[github-actions]

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