docs(howto): remove deprecated ee/ and .html extensions in observability backend guide

sai krishna chowdaryrequested to merge
kittusontineni/gitlab-development-kit:docs-observability-backend-canonical-links into main

What does this MR do?

Updates tracing, metrics, logs, ClickHouse, and SaaS simulation documentation links in doc/howto/gitlab_observability_backend.md to remove deprecated ee/ path prefixes and .html extensions, pointing to canonical documentation URLs.

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