docs: fix link to APM Overview

[bmorelli25]

In 7.16, the APM integration became generally available 🎉. As a part of this change, the APM Server Reference and APM Overview were deprecated and replaced with a new APM Guide. This PR updates a link in the ECS documentation to point to the new APM guide.

Unfortunately, when 7.17 is released, the old link will fail. To fix this, we need to backport this PR all the way to 1.3 😬 (which I'm happy to do). The broken links can be seen in elastic/docs#2333, but I've also included a summary below.

4:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.10/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.11/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.12/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.3/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.4/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.5/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.6/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.7/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.8/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/1.9/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/8.0/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/8.1/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/current/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html
14:17:47 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/ecs/master/ecs-products-solutions.html contains broken links to:
14:17:47 INFO:build_docs:   - en/apm/get-started/7.17/overview.html

[ebeahan]

LGTM 👍

I don't mind handling the backports; hopefully, the backport tool should simplify the process. 😃

[bmorelli25]

Thanks, Eric! I'll let you take care of it then 😄