Document Knowledge Hub content-tagging endpoints (Preview) (#556)

* Document public content-tagging endpoints in Preview OpenAPI spec

Add the four public v3 content-tagging endpoints to the Preview spec
(descriptions/0/api.intercom.io.yaml):

  POST   /articles/{article_id}/tags
  DELETE /articles/{article_id}/tags/{id}
  POST   /content_snippets/{content_snippet_id}/tags
  DELETE /content_snippets/{content_snippet_id}/tags/{id}

These document the implementation shipped in intercom/intercom#523830
(apply/remove an existing tag on articles + KB content snippets). The
endpoints are Preview-gated via the intercom_version_preview header,
reuse the existing tag/error schemas and the Unauthorized response, and
document the read_write_articles_scope / read_write_content_snippets
OAuth scopes plus the 403 (forbidden) and 404 (article/snippet/tag not
found) responses.

Companion to intercom/intercom#523830.

* Document internal-article tagging in Preview OpenAPI spec

Fold the internal-article tag support shipped in intercom/intercom#524047
into the content-tagging docs:

  POST   /internal_articles/{internal_article_id}/tags
  DELETE /internal_articles/{internal_article_id}/tags/{id}

Plus a version-gated 'tags' field added to the internal-article read
responses (GET /internal_articles and GET /internal_articles/{id}) via a
$ref to the shared 'tags' schema on internal_article_list_item, mirroring
public articles.

Write endpoints mirror the articles/content-snippets shape exactly: same
{id} body + optional admin_id, same AddContentTaggingApi Preview gate,
same VersionedTaggingPresenter response, same 403/404 errors (with an
internal_article_not_found case). Internal-article tags ride the
read_write_articles_scope OAuth scope (same as public articles), not a
separate scope.

Companion to intercom/intercom#524047 (and #523830 for articles + KB
snippets).

* Note Knowledge Hub content in tag applied_at/applied_by descriptions

Keep byte-identical with the developer-docs companion: extend the tag
schema's applied_at/applied_by descriptions to include articles, internal
articles, and content snippets returned by the content-tagging endpoints.

Author: kilian-tennyson