added embedding example

[pcoet]

Description of the change

Adding one more example to the LangChain.js node quickstart: Embedding text.

Motivation

This is a common use case and belongs in the quickstart.

Type of change

Choose one: (Bug fix | Feature request | Documentation | Other): Feature request

Checklist

• [ x] I have performed a self-review of my code.
• [ x] I have added detailed comments to my code where applicable.
• [ x] I have verified that my change does not break existing code.
• [ x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [ x] I am familiar with the Google Style Guide for the language I have coded in.
• [ x] I have read through the Contributing Guide and signed the Contributor License Agreement.

[MarkDaoust]

Looks totally reasonable, but I haven't tested.