doc: nrf-bm: libraries: add bm_storage documentation

[MirkoCovizzi]

Adds bm_storage library documentation.

[github-actions]

You can find the documentation preview for this PR here.

[eivindj-nordic]

As discussed offline I also wonder if the storage init API should take a configuration structure instead of setting parameters in bm_storage strut before calling init.

[bjorn-tore-taraldsen]

"must be a multiple of the backend’s program unit"
This leavs me with the question, what is the backend's program unit

[eivindj-nordic]

In the sample we have the comment

Write buffer size must be a multiple of the program unit. 
To support both RRAM (16 bytes) and SoftDevice (4 bytes) backends, that is 16 bytes.

Could probably be added to the documentation.

[bjorn-tore-taraldsen]

"multiple of the erase unit"
Definition of erase units, where is this defined

[b-gent]

Adding a commit with a bit of review, to be squashed before merging.

@MirkoCovizzi , can you add the following:

1. Add information about what we understand by 'program unit' and 'erase unit'.
2. Add doxygen doc in the header file for the 'len' parameter of the bm_storage_erase function (we mention it explicitly but it is simply not there).