Docs: Linear algebra introduction
Docs: Sparse Matrix Storage Formats
Docs: Matrix assembly
Docs: Building a sparsity pattern
Docs: add CSR diagram
Docs: include CSR diagram
Docs: add parallel matrix distribution diagram
Docs: remove title from parallel matrix distribution diagram
Docs: include parallel matrix distribution diagram
Docs: add matrix assembly diagram
Docs: make assembly diagram a bit more compact
Docs: insert assembly diagram
Docs: simplify sparsity building algorithms

Remove rowdim and coldim since we might switched to block matrices.
Docs: Add matrix assembly example
Docs: add section on solving a linear system
Docs: add section on GPU matrix assembly
Docs: add section references for backend docs
Docs: add generated code for matrix assembly par_loop
Docs: add CUDA matrix assembly and CSR conversion kernels
Docs: expand GPU linear algebra section