docs: InfluxDB shared steering docs

[fredjoonpark]

Expands the version-agnostic steering docs for the Timestream for InfluxDB Kiro power:

• getting-started.md — adds onboarding walkthroughs for both v2 and v3 (deploy, connect, client snippets for Python and curl)
• glossary.md — fills in terminology definitions
• influxdb-2-vs-3.md — adds detailed feature comparison, query language differences, and migration considerations
• line-protocol.md — adds full line protocol syntax reference, data types, escaping rules, and batch writing guidance