"Performance Tips" documentation

[mscroggs]

Describe new/missing feature

We should add a performance tips/pitfalls section to the docs, including:

1. Don't put form in loops
   • How to avoid it using dolfinx.fem.Constant and reassigning of dof-values in functions
2. Avoid data allocation inside loops
   • Explain sparsity pattern creation and matrix creation
   • Also is the same for dolfinx.fem.Function and PETSc.Vector
3. Don't create linear solver as it does all of the above (and creating the PETSc ksp object).
4. Usage of form compiler and jit options (cache control and optimization), ref https://jsdokken.com/dolfinx-tutorial/chapter4/compiler_parameters.html

(Suggestions for more things to include welcome)

Numba have something similar: https://numba.readthedocs.io/en/stable/user/performance-tips.html