Add `References` docstring sections with Markdown-style `[^key]`

[Luis-Varona]

Would help users actually see the references in the docstrings rather than having to go to the documentation site. See, e.g.,

  log(A::AbstractMatrix)

  If A has no negative real eigenvalue, compute the principal matrix logarithm of A, i.e.
  the unique matrix X such that e^X = A and -\pi < Im(\lambda) < \pi for all the
  eigenvalues \lambda of X. If A has nonpositive eigenvalues, a nonprincipal matrix
  function is returned whenever possible.

  If A is symmetric or Hermitian, its eigendecomposition (eigen) is used, if A is
  triangular an improved version of the inverse scaling and squaring method is employed
  (see [^AH12] and [^AHR13]). If A is real with no negative eigenvalues, then the real
  Schur form is computed. Otherwise, the complex Schur form is computed. Then the upper
  (quasi-)triangular algorithm in [^AHR13] is used on the upper (quasi-)triangular
  factor.

  │ [^AH12]
  │
  │  Awad H. Al-Mohy and Nicholas J. Higham, "Improved inverse scaling and
  │  squaring algorithms for the matrix logarithm", SIAM Journal on Scientific
  │  Computing, 34(4), 2012, C153-C169. doi:10.1137/110852553
  │  (https://doi.org/10.1137/110852553)

  │ [^AHR13]
  │
  │  Awad H. Al-Mohy, Nicholas J. Higham and Samuel D. Relton, "Computing the
  │  Fréchet derivative of the matrix logarithm and estimating the condition
  │  number", SIAM Journal on Scientific Computing, 35(4), 2013, C394-C410.
  │  doi:10.1137/120885991 (https://doi.org/10.1137/120885991)

[Luis-Varona]

Unfortunately, it's a lot harder to automatically generate the citations this way and render them in the bibliography blocks of the Documenter-generated website, so let's just stick to key for the in-text citations. Let's definitely add References sections to the docstrings, though.