ESQL: Docs: link `index.mode` page from `LOOKUP JOIN` page

[bpintea]

Description

The LOOKUP JOIN command docs read: LOOKUP JOIN enables you to add data from another index, AKA a 'lookup' index, to your ES|QL query results, simplifying data enrichment and analysis workflows., but there's no further hint on what a 'lookup' index is. We should link that to https://www.elastic.co/docs/reference/elasticsearch/index-settings/index-modules#index-modules-settings

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[leemthompo]

We should also link to the lookup join landing page from the function docs, I'm updating the landing page first because that's static page:

• [DOCS] Update esql-lookup-join.md to mention index mode requirement #126901 (9.x)
• [DOCS] [8.x] Update esql-lookup-join.asiidoc to mention index mode requirement #126905

[leemthompo]

@bpintea we should have links on every lookup join page now :)