Editable Document

You can set the sharing for the document to allow anyone to edit without signing in. Help

For a spreadsheet you can set a notification rule so you get a message when it is changed. There is no notification for documents, but there is a file history so you can revert to an earlier version of the file.

Visitors can edit the document by hovering over the document and clicking the icon to open it for editing.

Also you can embed a link to the Document. Link

Editable Document
Demo Spreadsheet

You can embed a spreadsheet so that no pop-out icon shows.

<iframe height="500" src="https://docs.google.com/spreadsheets/d/ID##########/edit?usp=sharing&embedded=true&rm=demo" width="75%"></iframe>

With this method viewers can edit it directly on the page. A side effect of this method is that the page automatically scrolls down to the document area. Note that the first click on the document will move to the top of the page and you must scroll back to it and click again. Google is aware of this problem, but they have not been able to fix it yet.

Idea posted by ChiefChippy2 here

It also works for an editable document

<iframe height="500" src="https://docs.google.com/document/d/ID#######/edit?usp=sharing&embedded=true&rm=demo" width="75%"></iframe>

Note that the first click on the document will move to the top of the page and you must scroll back to it and click again. Google is aware of this problem, but they have not been able to fix it yet.