[DOC] Add tutorial for adding your own objects

[smokestacklightnin]

This is a work in progress.

This is a continuation of PR #358. It was created by rebasing PR #358 to include the merged commits from PR #362 that removes max_input_size.

This PR adds a tutorial to help users or developers add assistants and source storages for LLMs to Ragna that are not currently supported.

TODO: Add usage example with the Python API