[DOC] Random number generation needs documentation.

### Is your feature request related to a problem? Please describe.

* Random number generation.
* How to use hardware random entropy source.
* What happens when hw random is not present.
* How security is impacted by random number generation (i.e. mbedtls).

### Describe the solution you'd like

Dedicated documentation chapter for a random number generation and implementation in NuttX.

### Describe alternatives you've considered

External information sources that may not match NuttX implementation.

### Verification

- [X] I have verified before submitting the report.