[assembler] Remove index for cloud, ecs, elasticsearch (#924)

Related to elastic/docs-builder#845

Remove some reference section landing pages:

* `reference/cloud/index.md`: Use the elastic/cloud repo's
[`reference/index.md`](https://github.com/elastic/cloud/blob/master/docs/reference/index.md)
instead
([preview](https://docs-v3-preview.elastic.dev/elastic/cloud/tree/master/reference))
* `reference/elasticsearch/index.md`: Use the elastic/elasticsearch
repo's
[`reference/elasticsearch/index.md`](https://github.com/elastic/elasticsearch/blob/main/docs/reference/elasticsearch/index.md)
instead
([preview](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/))
* `reference/ecs/index.md`: Use the elastic/ecs repo's
[`reference/index.md`](https://github.com/elastic/ecs/blob/main/docs/reference/index.md)
instead
([preview](https://docs-v3-preview.elastic.dev/elastic/ecs/tree/main/reference/))

Author: colleenmcginnis

reference/cloud/index.md

@@ -3,7 +3,6 @@ toc:
   - toc: security
   - toc: observability
   - toc: search
-  - toc: elasticsearch
   # The next one should be child of ☝️
   - toc: elasticsearch-clients
   - toc: ingestion-tools
@@ -12,8 +11,6 @@ toc:
   - toc: apm
   # The next one should be child of ☝️
   - toc: apm-agents
-  - toc: ecs
   - toc: data-analysis
-  - toc: cloud
   - toc: glossary