* Improve stale nessage

* Update .github/stale.yml

Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>

This need an option to be enabled on repo : https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository

Like the comment that is posted.

Ex: https://github.com/go-gitea/gitea/pull/4749 is closed after 2 weeks and not 2 months

For `UI`-bugs we usually require a Screenshot to debug it 😉

* Update example install url from try.gogs.io to try.gitea.io

* Ask if issue can be reproduced on try.gitea.io

* Link try.gitea.io to the README

The CONTRIBUTING.md from root dir will be used by github

I've noticed that a lot of issues cannot be reproduced on http://try.gogs.io,
which either hints about specific database type problems or
hints about bugs which are already solved in the newer version
(as http://try.gogs.io is usually a newer build).

This patch adds the suggestion to test the issue at http://try.gogs.io in
the Github "issue template". The user can answer: "Yes", "No", "Not relevant".

"Not relevant" is an option where testing on http://try.gogs.io makes no sense as
the bug is unrelated to the Web UI or is very specific in nature.