Add a link to the label descriptions in the issue templates

Description

I feel this is best place for people to review the labels for the project, and linking them from the issue template will make this easier to find, with the possibility they may apply utilise the labels more effectively after reading.

Changes proposed in this merge request:

• The issue templates no contain a link to the labels

BuildStream/nosoftware/alignment!8 (merged)

This merge request, when approved, will close:

---