Revise APM Instrumentation library injection details

[heyitsgrace996]

Updated the section on instrumentation library injection to specify Single Step Instrumentation and added information on manual APM tracer library injection.

What does this PR do? What is the motivation?

Motivation: We recently received feedback from a customer that this documentation is confusing and does not highlight the "APM and DogStatsD" section will only result in environment variables being injected, and not tracing SDKs. The instrumentation section also uses an old URL which now redirects to the SSI documentation, and does not clearly state the difference between SSI and local SDK injection methods.

Changes made:
The PR aims clarify the different APM configuration methods by doing the following:

• Updates the "Instrumentation library injection" and "APM and DogStatsD" section titles to highlight the former is for APM instrumentation, while the latter is for environment variable injection only.
• Update the "Instrumentation library injection" section to specify the current method is Single Step Instrumentation, and fix the old URL to the current SSI doc it was redirecting to.
• Add a second point which includes details and a link to the alternative APM injection method, Local SDK Injection (many customers prefer to use this over SSI, but struggle to locate the correct documentation).

Merge readiness:

• Ready for merge

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/grace.williams/apm_injection_update/containers/cluster_agent/admission_controller