fix(docs): make Node SDK quickstart and examples runnable (closes #51)

[justinmclean]

The Node quickstart sent to a stream and topic it never created, so it failed
immediately on a fresh server.

Both pages now:

• look up the stream and topic by name and create only what is missing, so runs
  are repeatable
• use Partitioning.Balanced instead of indexing topic.partitions, which is
  absent from the object topic.list returns even though topic.create
  includes it
• address the stream and topic by name in the consumer rather than assuming ids 1 and 1
• poll with PollingStrategy.Next and autocommit: true, so consecutive runs
  read successive batches instead of repeating the first ten
• fix partitionId from 1 to 0
• call client.destroy()

Verified against apache/iggy:edge: producer twice, consumer twice, second run
picking up the next batch.

AI was used to help draft this. Reviewed and tested by a real human.