docs: add mermaid graph for caching strategy

[goapunk]

Describe your changes Add a mermaid graph to visualize the cache flow

Tasks

• [ ] PR name contains story or task reference
• [ ] Steps to recreate and test the changes
• [x] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [ ] Changelog

[goapunk]

needs some introduction referencing the tool and how it is supposed to execute (run on their website) before the actual code.

@m4ra Hmm, I didn't use any tool (github renders it by default) and we don't have mkdocs in mB yet (which has a plugin for it). I can add a link to github and the link to the mermaid cli