Create a performance patterns section in the Anchor documentation

[nutafrost]

Introduce a dedicated performance section documenting escape hatches and optimization patterns available to developers who need to go beyond the default Anchor abstractions.

Content to include:

• Custom entrypoint with discriminator-based dispatch (as shown in the design doc) that falls back to the Anchor entry function.
• When and how to use raw AccountView access instead of typed Account<T> for hot paths.
• Zero-copy layout best practices: alignment, padding, repr(C) rules.
• Benchmarking methodology (CU usage, LiteSVM-based benchmarks).
• Trade-offs: what safety guarantees you give up with each escape hatch.

Acceptance criteria:

The Anchor documentation has a "Performance" section with runnable code examples and CU comparisons.