docs: Add explanation of cost model

[j-mueller]

I started writing this last week. I don't have any more time to work on it right now but maybe it can be merged as it is.

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• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Commit sequence broadly makes sense
  • [ ] Key commits have useful messages
  • [ ] Relevant tickets are mentioned in commit messages
  • [ ] Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • [ ] Self-reviewed the diff
  • [ ] Useful pull request description
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[sjoerdvisscher]

What's the status of this PR?

[j-mueller]

It can be merged if @michaelpj is happy with it