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I don’t think there is a way to do it built into Docs. There is an add-on called VexTab that has ok reviews. It also support tabliture, which might be of interest to you judging from your avatar. Open a Doc file, and click the “add-on” tab at the top of the page, then select “get add-ons”. You can search for it using the search box in the top right of the pop-up.