[docs] move all references to bibliography.rst

[sdrave]

Fixes #475.

[sdrave]

The results can be seen here.

[pmli]

Looks good. I especially like that most of the references are systems-theoretical 👌

[codecov]

Codecov Report

Merging #484 into master will increase coverage by 0.33%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
src/pymor/algorithms/lyapunov.py	74.82% <ø> (ø)	⬆️
src/pymor/reductors/interpolation.py	76.63% <ø> (ø)	⬆️
src/pymor/reductors/parabolic.py	95.74% <ø> (ø)	⬆️
src/pymor/reductors/bt.py	53.9% <ø> (ø)	⬆️
src/pymor/reductors/sobt.py	81.43% <ø> (ø)	⬆️
src/pymor/reductors/coercive.py	76.92% <ø> (ø)	⬆️
src/pymor/reductors/sor_irka.py	65.68% <ø> (ø)	⬆️
src/pymor/algorithms/adaptivegreedy.py	83.62% <ø> (ø)	⬆️
src/pymor/reductors/h2.py	65.56% <ø> (ø)	⬆️
src/pymor/core/defaults.py	71.32% <0%> (-13.41%)	⬇️
... and 3 more