Improve bug reporting mechanism

[oddhack]

Followon to #46. We now have a site UI navbar 'Feedback' menu which links off to the Vulkan-Site new issue UI, but this could use additional context to help people report:

• Should the navbar menu item open in a new window instead of replacing the docs.vulkan.org view in the current browser window? I think so - in general people will probably report from the page showing the problem.
• There should be at least one bug-reporting template in the Vulkan-Site repo which directs people in how and where to report. Ideally problems with component content would be reported in those repos, while problems with the site itself and interrelationships between components would be reported here. People won't always get this right. Also the usual boilerplate like (where did you see the problem, what were you doing, what browser, what did you get, what did you expect, what do you want done about it).

[oddhack]

This is fixed by the UI PR above, which we should sign off on ASAP - it is straightforward.