Document RAM measurements

[yanesca]

Compile and run the benchmark (programs/test/benchmark.c) application in the following configuration:

#define MBEDTLS_PLATFORM_C
#define MBEDTLS_PLATFORM_MEMORY
#define MBEDTLS_MEMORY_BUFFER_ALLOC_C
#define MBEDTLS_MEMORY_DEBUG

Hammer out any potential problems, decide about the RAM measurement metrics to track and document it in kb/how-to/non-functional-testing.md (create the file if it doesn't exist yet).