[documentation] Add a page about the foundations of Krylov methods

[amontoison]

I could give a link to this page to justify my message in this discussion: https://discourse.julialang.org/t/how-to-solve-this-ax-b-faster/81693/22

[codecov]

Codecov Report

Merging #593 (526f74f) into main (80bea76) will not change coverage.
The diff coverage is n/a.

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[github-actions]

Package name	latest	stable
CaNNOLeS.jl
DCISolver.jl
JSOSolvers.jl
LLSModels.jl
Percival.jl
RipQP.jl

[dpo]

Are the docs the right place to add such background? The background only talks about square systems. I think that if you really want some background in here, it should be less specific and more general: Krylov methods build approximations by constructing subspaces of increasing dimension and choosing the approximations using an optimization problem.