Do you want to use it in your own application? How do you want to send the email? You have several options.
a) You could indeed add it to portal-ext.properties, but I would not recommend that. It will be messy, since you should just add some short config options there, not whole documents
b) You could use a webcontent article for it. Then you would only need to store the ID of the webcontent somewhere. Editors can then just edit the article and you would only need to read it using the API where you send the email.
c) You could add a configuration. You will only have a text editor there, but it could be sufficient if you don't plan to do something fancy.
https://help.liferay.com/hc/en-us/articles/360018161391-Making-Your-Applications-Configurabled) You could create your own service to store the email template. This approach is pretty powerful, but if you need it only once or twice, it probably doesn't pay off.
