Update performance docs to take TPC into account

We have an [inheritance mapping](https://docs.microsoft.com/en-us/ef/core/performance/modeling-for-performance#inheritance-mapping) section in the perf docs which compares TPH and TPT. Aside from adding TPC into the mix, we should consider moving that to the [modeling page on inheritance](https://docs.microsoft.com/en-us/ef/core/modeling/inheritance) (and linking from the perf docs) - perf should be an essential factor in the decision of which strategy to use.

Also point sparse columns to [our own docs](https://docs.microsoft.com/en-us/ef/core/providers/sql-server/columns#sparse-columns) rather than to the SQL Server ones.