Adding some github customization.

[Andy Seaborne]

Now we are using github issues and discussions, we can consider 
customizing it a bit.

Issue:
https://github.com/apache/jena/issues/1362
PR:
https://github.com/apache/jena/issues/1363
You can try the issue templates out at https://github.com/afs/templates/ 
(temporary).

This is not to preclude receiving pull requests, issue and bug reports 
by other channels such as JIRA.

Comments please on the concept and the content.

   Andy