OBSDOCS-3137 Cut DT content from main and point to standalone docs

[gtrivedi88]

Summary

• Remove all DT-only modules (34), snippets (5), and assemblies (6) from main now that standalone DT docs are published
• Convert the architecture assembly into a single pointer page linking to the standalone docs at docs.redhat.com
• Strip DT topic map entries from 7 to 1 (pointer page only)
• Convert xrefs to deleted DT assemblies into external links in observability/overview/index.adoc, welcome/learn_more_about_openshift.adoc, service_mesh/v2x/ossm-observability.adoc, and serverless/observability/tracing/serverless-tracing.adoc
• Keep 3 modules + 1 snippet that are shared with Service Mesh and Serverless docs

Follows the same pattern as the OTel cut in PR #107422 (OBSDOCS-2987).

Test plan

• Verify _topic_map.yml builds correctly with only the pointer page entry
• Verify the pointer page renders with the external link to standalone docs
• Verify Service Mesh and Serverless pages that reference shared DT modules still build
• Verify no broken xrefs remain in the repo
• Cherry-pick to enterprise-4.12 through enterprise-4.22

[ocpdocs-previewbot]

🤖 Tue Mar 10 14:40:40 - Prow CI generated the docs preview:
https://108143--ocpdocs-pr.netlify.app
Complete list of updated preview URLs: artifacts/updated_preview_urls.txt

[openshift-ci]

@gtrivedi88: all tests passed!

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