add a description field for external doc links

[miguelgrinberg]

Fixes #4225

This change adds a fourth column to table.csv where a description for an external documentation link can be given. This description is included in schema.json, and later transferred to the externalDocs object in the generated OpenAPI files.

Note that I have not added any descriptions, the 4th column in table.csv is empty for all rows in this PR.

[lcawl]

To test this PR, I added a description to one of the rows in the table.csv:

ccr-auto-follow,https://www.elastic.co/docs/deploy-manage/tools/cross-cluster-replication/manage-auto-follow-patterns,https://www.elastic.co/guide/en/elasticsearch/reference/8.18/ccr-auto-follow.html,Auto-follow pattern documentation

Then I generated the output and verified that it appeared in our final OpenAPI document:

  "/_ccr/auto_follow/{name}": {
      "get": {
        "summary": "Get auto-follow patterns\n",
        "externalDocs": {
          "description": "Auto-follow pattern documentation",
          "url": "https://www.elastic.co/docs/deploy-manage/tools/cross-cluster-replication/manage-auto-follow-patterns",
          "x-previousVersionUrl": "https://www.elastic.co/guide/en/elasticsearch/reference/8.18/ccr-get-auto-follow-pattern.html"
        }
...

And confirmed that the new link text appears in a preview of these docs:

[lcawl]

Per #4772 (comment) the output LGTM, thanks!