[SVLS-7119] add instructions to install Serverless monitoring for .NET Azure Functions

[duncanpharvey]

What does this PR do? What is the motivation?

Adds instructions to install Serverless monitoring for .NET Azure Functions.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

• Rewords installation instructions to be consistent across languages
  • Start the Datadog Serverless Compatibility Layer and initialize the Datadog <language> tracer
• Fixes links to runtime metrics and custom metrics documentation
• Removes references to specific tracer versions. Recommending the latest version is sufficient now that automatic instrumentation of http triggers has been available for several months
• Removes instructions for trace stats. DD_TRACE_STATS_COMPUTATION_ENABLED and DD_TRACE_TRACER_METRICS_ENABLED are for a configuration that is no longer maintained
• Removes note around Linux Consumption plans and GitHub Actions since recent fixes have remedied this issue

[github-actions]

ℹ️ Documentation Team Review

No documentation team review is required for this pull request.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/duncan-harvey/azure-functions-dotnet-instrumentation/serverless/azure_functions/

[lucaspimentel]

Probably fine for a first pass, but we should consider consolidating all the env vars later. Right now the steps are

3. add env vars
4. deploy your func
5. add more env vars

[duncanpharvey]

Let me know if you have any formatting recommendations for this! We have the .NET specific environment variables in the programming-lang lang block and the generic environment variables outside of it so I'm not sure the best way to reconcile those two things.