I had the condition myself. While it may be painful, you aren’t likely to do permanent damage to your knee. Stay away form activities that cause pain, obviously. That being said, training around knee problems basically involves avoiding exercises that use the quadriceps beyond a shallow knee bend, as the quadriceps insert via the patella.
In other words, no squatting, but things like RDL’s or single leg RDLs that use only a very shallow knee bend will be okay. I had patellar tendinitis freshman year of college and couldn’t do a bodyweight squat without pain, but I could RDL 350+ lbs for reps because there is virtually no quad involvement. Glute ham raises and forward sled drags are great options as well. Leg curls should be fine, but avoid leg extension, lunges or any type of exercise that specifically targets the quads. You will be hammering the glutes and hamstrings for some time, and may be able to tolerate light bodyweight squats or lunges if the condition improves.
All this being said, consult with your doc first and make sure everyone is on the same page.
Just my .02