Create a dedicated repository for binary documentation assets?

In light of issues such as #169, I wonder if we should establish a way to store binary blobs (aka image files) that we want to use in the documentation. Right now, we already use 2-3 images in the [README.md](../blob/master/README.md) and the docs, but each time I consider adding a new one I feel slightly bad about permanently increasing the repo size by 50-100 KiB. This is exacerbated by the fact that images, just like other documentation, often become outdated very fast and need to be replace regularly.

One possible solution could be to have a dedicated repository to store documentation assets such as images or videos, e.g., `trixi-framework/documentation-assets`, that we allow to grow in size over time. A different approach could be to store binary assets in a dedicated *branch* within the main repo, which gets regularly purged by deleting its history (however, this would raise the question of if and how docs for older releases should be kept alive).

Another thought (although slightly orthogonal to the main question here), is whether it would be beneficial to have a dedicated repository to host the documentation, such as `Oceananigans.jl` does it with [OceananigansDocumentation](https://github.com/CliMA/OceananigansDocumentation).

Review README.md and the current documentation image references, then read issue #169 for related context. Compare the proposed dedicated repository, a dedicated branch, and a separate documentation repository, including how existing and older-release assets would be preserved; done means reaching and recording a clear project decision.

Written by the indexing model from the issue text.