Improved Getting Started Documentation

## Improvement Suggestion

In https://iceoryx2.readthedocs.io and also our examples we need to clearly define the restrictions on the payload type. On the Rust side we can solve this cleanly with the `shmem` derive macro but on the C/C++ side we must document it.

It already led to a bug; see #436, so this shall be done sooner rather than later. Otherwise, another user will also run into this pitfall.

Review the Getting Started documentation at https://iceoryx2.readthedocs.io and the repository examples, then inspect the shmem derive macro and issue #436 for the payload restriction context. Done means clearly documenting the allowed payload types and restrictions for Rust, C, and C++ users in both the documentation and examples.

Written by the indexing model from the issue text.