Add arxmliv subresource page for bibtex url to resolve

I just realized our official bibtex for the datasets points to a URL that does not exist:

@MISC{SML:arXMLiv:08.2018,
  author = {Deyan Ginev},
  title = {arXMLiv:08.2018 dataset, an HTML5 conversion of arXiv.org},
  howpublished = {hosted at \url{https://sigmathling.kwarc.info/resources/arxmliv/}},
  note = {SIGMathLing -- Special Interest Group on Math Linguistics},
  year = 2018}

So, I am adding that placeholder page, without linking it anywhere, as the resource has already been cited, so we should facilitate the URL. I will make sure to use the specific dataset URL for the 2019 bibtex entry.