Add Performance & Latency Patterns section (Local Activities, Early Return + Local Activities, Eager Workflow Start)

[joshmsmith]

Adds a new "Performance & Latency Patterns" section with three complementary patterns for reducing Temporal workflow latency:

• Local Activities — eliminates server round-trips (~50 ms per activity saved), all 4 languages
• Early Return + Local Activities — combines Update-with-Start with local Phase 1 for ~160 ms first-response latency, all 4 languages
• Eager Workflow Start — bypasses the Matching Service on workflow start (~30–50 ms saved), Go/Java/Python

Also fixes:

• mapreduce-tree Java: correct newUntypedExternalWorkflowStub API call
• sliding-window Java: remove unused HashSet import