docs: fix broken links and examples

[Bortlesboat]

This picks up a few verified docs regressions on current main.

Changes:

• fix stale mcp-usage.md anchor links in vpc-flow-analysis.md
• normalize the BigQuery MCP tool name to execute_sql across the networking observability references
• repair the malformed multiline gcloud run deploy example in the onboarding recipe

Verification:

• ran a local markdown-target check to confirm the updated mcp-usage.md anchor targets exist
• confirmed the touched networking observability files no longer mention query_sql or execute_sql_readonly
• confirmed the onboarding recipe no longer contains the broken \ --platform=managed continuation

Closes #33

I realize CONTRIBUTING.md says external PRs may not be accepted; sharing this as a ready-to-apply patch in case it helps with the internal sync path.