[DOCS] Fix hard-coded links to master

[lcawl]

Relates to elastic/docs#3160

This PR fixes broken links like:

INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/serverless/main/what-is-elasticsearch-serverless.html contains broken links to:
--
  | INFO:build_docs:   - en/kibana/master/playground.html

INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/serverless/main/elasticsearch-playground.html contains broken links to:
--
  | INFO:build_docs:   - en/kibana/master/playground.html
  | INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/serverless/main/general-ml-nlp-auto-scale.html contains broken links to:
  | INFO:build_docs:   - en/elasticsearch/reference/master/infer-service-elasticsearch.html
  | INFO:build_docs:   - en/elasticsearch/reference/master/infer-service-elser.html
  | INFO:build_docs:   - en/elasticsearch/reference/master/put-inference-api.html
  | INFO:build_docs:   - en/elasticsearch/reference/master/start-trained-model-deployment.html
  | INFO:build_docs:   - en/elasticsearch/reference/master/update-trained-model-deployment.html

I think this PR only needs to be merged if we assume that we'll want to stop the main branch of the Kibana Guide before we stop building the only branch of the Serverless Guide.

[leemthompo]

LGTM, thanks Lisa!