Explain how to store documents in local storage

[hackerwins]

Description:

Explain how to store documents in local storage.

Users can edit the document even while offline. However, because the document is stored in memory, all changes are lost when the users close the browser.

Let's explain how to store the document's snapshot and local changes in local storage so that the edits remain even when the users close the browser.

Related to #364.

Why:

• Help developers implement storing documents in local storage.