Add `refs.bib`

[purva-thakre]

Combine articles.bib and books.bib into a ref.bib.

Remove the following and add as an in-line explicit link.

toqito/docs/articles.bib

Lines 214 to 217 in 49ecfd4

	@misc{Riley_2022_CVXPYKron,
	author = {Murray, Riley J},
	title = {{PR:} Have kron support non-constant expressions in either argument from {CXPY:} A Python-embedded modeling language for convex optimization problems},
	howpublished = {https://github.com/cvxpy/cvxpy/issues/457#issue-309891424}}

Previously discussed here: #614 (comment)