Documentation: removed Communicators from the core concepts, MPI will be documented separately

   (TODO: rename to `Executor` or something like that)

   - Device is responsible for the execution of algorithms in a specific way.

   - Algorithms can be specialized by the `Device` template parameter.

3. \ref TNL::Communicators "Communicators"

   - Communicators represent the main abstraction for distributed computations,

     where multiple programs (or instances of the same program) have to

     communicate with each other.

   - At present, there are only two communicators:

     \ref TNL::Communicators::MpiCommunicator "MpiCommunicator"

     (uses [MPI](https://en.wikipedia.org/wiki/Message_Passing_Interface)) and

     \ref TNL::Communicators::NoDistrCommunicator "NoDistrCommunicator"

     (dummy communicator without any distribution support).

4. \ref TNL::Containers::Algorithms "Algorithms"

3. \ref TNL::Containers::Algorithms "Algorithms"

   - Basic (container-free) algorithms specialized by `Device`/`Executor`.

   - `ParallelFor`, `Reduction`, `MultiReduction`, `ArrayOperations`, ...

5. \ref TNL::Containers "Containers"

4. \ref TNL::Containers "Containers"

   - Classes for general data structures.

     (TODO: alternatively use "Dense" and "Sparse", because a dense matrix can

     be an extended alias for 2D array)

   - `Array`, `Vector` (also `VectorOperations`), `NDArray`, ...

6. Views

5. Views

   - Views wrap only a raw pointer to data and some metadata (such as the array

     size), they do not do allocation and deallocation of the data. Hence, views

     have a fixed size which cannot be changed.

   - 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

6. Smart pointers

   - TODO

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