Breastmilk yogurt is just another form of breastmilk and isn't introducing solids. Introducing "solids" means introducing complementory foods. People say solids because complementory is a hard word for most people to understand and spell. Some moms start with jar food, some make their own food that they spoon feed, and some mash food with a fork and let the baby self-feed.
You don't have to have a doctor's okay to start solids if your baby was full term and healthy. Doctors usually have little training in nutrition in nutrtion or breastfeeding. It is better to find information from people that have experience and expertise and organizations that are known to have good information.
Here are instructions for breastmilk yogurt.
Here is more info than you ever want to have on starting solids
Kellymom is always a good place to look for information. There was an article in Mothering Magazine a couple of years ago about the importance of starting a wide variety of foods early on so your baby would like a lot of foods. I couldn't find it searching quickly. Maybe someone else remembers it.
There has been recent (last 10 years) research and articles that you can find on the AAP website. What they are saying is that there is no reason to delay starting foods that are likely to cause alleries including peanuts in the first year. Still no cow or goat or any other mammal milk the first year. There is no evidence that not feeding babies these foods helps prevent allergies.