Glad to see that you are getting interested in making addons!

First off, I don't believe what you are asking for would be possible. An addon cannot tell the health of an object unless the player is looking at it (like its on your target frame, the thing at the top of the screen).
That being said, you can make it so that if you look at the door and it has less than 20% health, then the message will pop up.

Heres a list of useful resources that will help you out:
The esoui wiki (The most useful link as it has tutorials and descriptions and everything.)
ESO's API
ESO's Event List

Baertram's post on tips to help addon making
ESOUI Forums
Additional Tools that may help


ESO's addons use lua, which is one of the easier languages to pick up in my opinion.

How I would go about doing this:
Register an event with local event = EVENT_MANAGER:RegisterForEvent() and the event being EVENT_POWER_UPDATE
Either add a filter to the event or use if statements in the event callback to make sure that the function only runs if the unit is the target.
In the callback, add another check to test if the name of the target via GetUnitName() is Keep Door
Add one more check to test if the health is under 20%
Finally, if all conditions are met, show something on-screen with something like announcements



I would go and work on this myself, but I'm pretty busy fixing bugs with my own addon. If I have spare time though, I'll see if I can implement this into my addon.