Documentation improvements

[scttfrdmn]

Goals

• Ensure all functions have complete Roxygen documentation
• Add more examples to function docs
• Improve getting started guide
• Add troubleshooting section

Tasks

• Review all exported functions
• Add @examples to key functions
• Expand troubleshooting guide
• Add FAQ section

[scttfrdmn]

Status (triage): partially addressed. getting-started, troubleshooting, security, performance, and 8 worked examples are published (https://starburst.ing). Keeping open for the remaining checklist items (comprehensive @examples coverage, FAQ). Note: a follow-up is warranted to guard the \donttest{} examples and vignettes the same way the integration tests were just gated (PR #25) — they currently execute live starburst_*() calls under R CMD check when AWS creds are present.