Reorganize tutorials, continued

[melissawm]

This PR addresses the following items of #24632:

• Move “Controlling data and computation placement on devices” from FAQ to Intro to parallel programming
• Split “Custom Pytrees” off of the pytrees tutorial and into its own Advanced section
• Move “Buffer donation” from FAQ to a new section under “Performance optimizations”
• Move “Benchmarking” from FAQ to a new section under “Performance benchmarking and profiling”