Seed user guide from existing google doc

https://docs.google.com/document/d/1xb5VW09DgFeNxyMCVYg_uo_torqCBqEE0Uxzl_11OtA/edit?usp=sharing has a section at the bottom with some useful stuff to copy over.

We can start more sections under https://a-b-street.github.io/docs//user/index.html

The issue references a Google Doc with content to copy. Start by reviewing the linked document and the existing user guide structure at https://a-b-street.github.io/docs//user/index.html. Determine where the new sections fit, then add the content to the appropriate Markdown files in the repository. Verify the changes build correctly to update the live site.

索引モデルが issue の本文から書いたものです。