[docs][infra] Integrate the `sphinx-issues` extension

[webknjaz]

Apparently, this extension implements many of the extlinks defined in docs/conf.py: https://github.com/sloria/sphinx-issues#usage-inside-the-documentation. I think, it's always better to reuse a single implementation than to keep copying/synching extlinks across projects. Besides, it's a bit more clever, than regular extlinks.

[gaborbernat]

PR welcome 👍