2.20 documentation + link issues

[conradsnicta]

@rcurtin Looks like the recent 2.20 release has a few minor issues.

• The changelog on the documentation page (https://ensmallen.org/docs.html#historychangelog) currently shows a bunch of ??? marks instead of numbers:

  ensmallen ?.??.?: “???”
  ????-??-??

• The release section on the right hand side of the main GitHub page (https://github.com/mlpack/ensmallen) shows version 2.19.1 instead of 2.20.0