Improve listing of index mode options in docs

[ioanatia]

currently there is no visual break between the options for index.mode and index.routing_partition_size which is an index setting:

checking to see if this change makes it better (looks good in my editor), but in the end it depends on how the docs are actually rendered.

EDIT: this is how it looks in the preview

[elasticsearchmachine]

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

[leemthompo]

Thanks for catching this @ioanatia. I think there is also an indentation issue with nested definition lists here. It's a little finicky.

I noodled on this locally, if you're happy with this output I can push a commit :)

Note the example block and subsequent text are indented.

I also added subheadings to clarify sub-sections and link to lookup join.

[ioanatia]

@leemthompo feel free to push a commit

[leemthompo]

@ioanatia done, can see preview here: https://docs-v3-preview.elastic.dev/elastic/elasticsearch/pull/127155/reference/elasticsearch/index-settings/index-modules

[leemthompo]

Thanks for opening the PR @ioanatia 🥇

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	9.0	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 127155