Documentation: add programming principles and smart pointers to the core concepts

   - Views have a copy-assignment operator which does a deep copy.

   - Views have all other methods present in the relevant container (data

     structure).

7. Smart pointers

   - TODO

TODO: formalize the concepts involving lambda functions (e.g. in `Reduction`)

## Programming principles

TNL follows common programming principles and design patterns to maintain a

comprehensible and efficient code base. We highlight some principles with

respect to the support for different compute architectures:

- CUDA kernels should not operate with needlessly extensive objects, e.g.

  objects which include smart pointers, because this wastes the device

  registers.

- CUDA kernels should not operate with "distributed" objects – they should

  operate only with the "local parts" of the distributed objects. MPI support is

  a higher layer than CUDA support and distributed objects generally contain

  attributes which should not be needed by CUDA kernels.

- Smart pointers should be cached if appropriate in order to avoid repeated

  memory allocations and copies.