Add a diagram for parts.rst
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Current evaluation
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Issue body
This PR adds a diagram to the "parts" doc.
Edit a live copy of the diagram [here](https://mermaid.live/edit#pako:eNp9kd1qhDAQhV8lDL1cX8CLglVbLLYr0RYKAZndTF1pjBKTwnZ33712fyyytXeT-c6cM2R2sG4lgQ-VwW7DUi600L1bnZ4ZGiv0TciD-6LMAl6UOQ-njbuXJI2mreQ5L4I0FZq0_LG7ML58jMOijBLOhPY873bfOaX27Mr_yFauVnIKz1lzeMw9CnqLFY2CgTzEl1RTN78k48lT_PeaZ337SUbhdpxYvsY8Dd7-y5n5kJOdqVDXXzSzPCygIdNgLYer7IRmTIDdUEMC_KGUaD4ECH0YdOhsm2_1GnxrHC3AdRItRTUO12vAf0fV0-EbhoebEQ).
Evaluation history
No evaluation history available.