Clean up broken links and unused variables in docset.yml files

[lcawl]

Description

This issue kind of deals with three different problems, but if we're going to have to touch every repo that contains MD files again, it makes sense to deal with all three at once:

1. There are some asciidoc-style links that were not converted to V3-style links. For example: https://www.elastic.co/guide/en/cloud/current/ec-add-user-settings.html[user setting] They seem to often be surrounded by parentheses, but can likely be found mostly easily by searching for ".html[" in *.md files. They should instead be updated to use the following format:
   https://docs-v3-preview.elastic.dev/elastic/docs-builder/tree/main/syntax/links.
2. Some of these broken links exist in the substitutions defined in the docset.yml files. Since all of our docset.yml files currently have the same list of substitutions, I think it means every repo that we've migrated docs into will be affected.
3. Per chat with @colleenmcginnis and @Mpdreamz if we're touching all those files anyway, it's a good time to remove all the variables that are unused in each repo. That way we don't have to unnecessarily update multiple docset.yml files in the future for substitutions that aren't even used there.

Resources

Please check off the repo in this list if you've confirmed that either the problems don't exist in that repo or that they're fixed (e.g. by adding commits to existing PRs in some cases)

• apm-agent-android (waiting on Docs update apm-agent-android#406)
• apm-agent-dotnet [docs] Miscellaneous docs clean up apm-agent-dotnet#2576
• apm-agent-go [docs] Miscellaneous docs clean up apm-agent-go#1705
• apm-agent-ios [docs] Miscellaneous docs clean up apm-agent-ios#261
• apm-agent-java [docs] Miscellaneous docs clean up apm-agent-java#4012 - Some broken URLs exist, but are embedded in code blocks and so I don't think we can fix until we know the final URLs
• apm-agent-nodejs No broken links
• apm-agent-php [docs] Miscellaneous docs clean up apm-agent-php#1295
• apm-agent-python [docs] Miscellaneous docs clean up apm-agent-python#2240 - Broken URL exists only in docset.yml, will be fixed when Colleen opens PR to clean up unused substitutions?
• apm-agent-ruby [docs] Miscellaneous docs clean up apm-agent-ruby#1526
• apm-agent-rum-js [docs] Miscellaneous docs clean up apm-agent-rum-js#1588
• apm-aws-lambda [docs] Miscellaneous docs clean up apm-aws-lambda#630
• apm-k8s-attacher [docs] Miscellaneous docs clean up apm-k8s-attacher#142
• beats [docs] Miscellaneous docs clean up beats#43370
• cloud-on-k8s [docs] Miscellaneous docs clean up  cloud-on-k8s#8558
• cloud
• curator
• docs-content Fix asciidoc-style link #858
• ecctl [docs] Miscellaneous docs clean up  ecctl#712
• ecs-dotnet [docs] Miscellaneous docs clean up ecs-dotnet#487
• ecs-logging-go-logrus [docs] Miscellaneous docs clean up ecs-logging-go-logrus#58
• ecs-logging-go-zap [docs] Miscellaneous docs clean up ecs-logging-go-zap#91
• ecs-logging-go-zerolog [docs] Miscellaneous docs clean up ecs-logging-go-zerolog#29
• ecs-logging-java [docs] Miscellaneous docs clean up ecs-logging-java#314
• ecs-logging-nodejs [docs] Miscellaneous docs clean up ecs-logging-nodejs#210
• ecs-logging-php [docs] Miscellaneous docs clean up ecs-logging-php#82
• ecs-logging-python [docs] Miscellaneous docs clean up ecs-logging-python#164
• ecs-logging-ruby [docs] Miscellaneous docs clean up ecs-logging-ruby#67
• ecs-logging [docs] Miscellaneous docs clean up ecs-logging#90
• ecs
• eland
• elastic-serverless-forwarder [docs] Miscellaneous docs clean up elastic-serverless-forwarder#875
• elasticsearch-hadoop [docs] Miscellaneous docs clean up elasticsearch-hadoop#2366
• elasticsearch-java [docs] Miscellaneous docs clean up elasticsearch-java#965
• elasticsearch-js [docs] Miscellaneous docs clean up elasticsearch-js#2663
• elasticsearch-net [docs] Miscellaneous docs clean up elasticsearch-net#8478
• elasticsearch-php [docs] Miscellaneous docs clean up elasticsearch-php#1437
• elasticsearch-py [docs] Miscellaneous docs clean up elasticsearch-py#2853
• elasticsearch-rs [docs] Miscellaneous docs clean up elasticsearch-rs#248
• elasticsearch-ruby [docs] Miscellaneous docs clean up elasticsearch-ruby#2604
• elasticsearch [DOCS] Clean up Asciidoc links in markdown files elasticsearch#125046
• go-elasticsearch [docs] Miscellaneous docs clean up go-elasticsearch#971
• integrations [docs] Miscellaneous docs clean up integrations#13182
• kibana [docs] Miscellaneous docs clean up kibana#215260
• logstash-docs
• logstash [docs] Miscellaneous docs clean up logstash#17372 Some broken URLs exist, but are embedded in code blocks and so I don't think we can fix until we know the final URLs
• search-ui [docs] Miscellaneous docs clean up search-ui#1127
• integration-docs
• security-docs [docs] Miscellaneous docs clean up security-docs#6646

NOTE: Warnings about unused substitutions were added per elastic/docs-builder#762 but there might be a bug in that per elastic/docs-builder#779

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

N/A

What release is this request related to?

9.0

Collaboration model

The documentation team

Point of contact.

N/A