[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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.05%. Comparing base (80bea76) to head (526f74f).
⚠️ Report is 424 commits behind head on main.

Additional details and impacted files

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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.