[docs][infra] Integrate the sphinx-issues extension #3202
Description
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.