Android - How can I add a shortcut to a particular Google Drive document to my Android homescreen?

You should be able to do this natively.

Android 3.0 (and below):

In your launcher, press and hold on a blank area and click "Shortcuts". Select "Drive Shortcut", then your document, and it will create a link for you.

Android 4.0 (and above)

Open your app drawer and jump to the Widgets section. Swipe until you get to "Drive Shortcut". Long-press and drop on your desktop. Choose the account to use (if necessary) then select the document.

Here is a slightly different take on Mike's answer, with screenshots:

_{(Click an image to enlarge it)}

1. Open the Drive app (not the app for the specific document type; this won't work in the Docs, Sheets, Slides, etc. apps)

2. Click the 3 dots (or touch-and-hold) to open the context menu on the document

   

3. Scroll down if needed and click "add to homescreen"