[2.x] docs: documentation restructure (#1428)

[bmorelli25]

Backports:

• docs: documentation restructure ([docs] Documentation restructure #1428)

[bmorelli25]

Yikes. This going to take some work in the APM Server repo.

14:59:55 INFO:build_docs:Bad cross-document links:
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/7.0/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/7.1/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/7.2/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/7.3/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/7.4/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/current/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started
14:59:55 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/apm/get-started/master/agents.html:
14:59:55 INFO:build_docs:   - en/apm/agent/nodejs/2.x/intro.html#get-started

[bmorelli25]

Opened all the PRs. Don't feel like chasing the build into the evening if I break it, so I'll merge everything tomorrow morning.