Document clearly how Mbed Crypto provider keys are stored

[ionut-arm]

The documentation for what gets stored on disk for the Mbed Crypto provider isn't clear enough, as it could be misunderstood (based on the store_path of the KeyInfoManager) that the keys are stored along with their mappings.

In reality, the keys are stored locally in the working directory of the process using the Mbed Crypto library. This can be changed at compile time (see this) - maybe we could also include this as part of our build, but the main point is that the behaviour should be described more thoroughly somewhere, preferably both in the book and in the config file

[hug-dev]

Related to #53

[ionut-arm]

Solved in the two PRs linked above