Update architecture diagrams

One of the actions out of the CSET day in January was to update the architecture/C4 diagrams and host these on the documentation pages, so help users/new contributors understand the architecture of CSET.

Start by locating the existing architecture/C4 diagrams and the documentation pages where they should be hosted. Review how current documentation assets are organized, then update the diagrams and confirm they are available on the documentation pages for users and new contributors.

Written by the indexing model from the issue text.