[Website]: Document join field type as a difference on Serverless

[ppf2]

Type of issue

Missing information

What documentation page is affected

https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings

What happened?

Join field type is not available on Serverless, while https://www.elastic.co/docs/reference/elasticsearch/mapping-reference/parent-join does have the unavailable tag, let's also document this on the differences page. Thx!

Additional info

No response

[shainaraskas]

context: https://elastic.slack.com/archives/C0D8ST60Y/p1748327726478049