Create java-manual-instrumentation.md

[janewang-datadog]

What does this PR do? What is the motivation?

Creates a new page on how to set up Data Streams Monitoring through manual instrumentation. Can the reviewer help hide this page from the index? Thanks in advance!

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/janewang-datadog/manual-instrumentation/data_streams/java_manual_instrumentation

[jhgilbert]

Thanks, @janewang-datadog! I created a docs ticket for the editorial review.