Explained where create issue/PR template (#29035) (#29266)

Explained where create issue/PR template (#29035) (#29266)

For some user (as me), documentation lack of precision about where to
store issue/pr template.

I propose an enhancement about this point. With bold exergue and
precision about server itself.

I've found some user with same interrogation as :
https://forum.gitea.com/t/issue-template-directory/3328

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Co-authored-by: Km <cam.lafit@azerttyu.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>