docs: add bug report and extension proposal issue templates

[chrisdebian]

Adds two GitHub issue forms, replacing the blank default:

• Bug report — structured fields for backend (yserver/ynest), GPU/driver,
  distro/kernel, and desktop environment, matching the detail level real bug
  reports already tend to include (e.g. Steam Library tab crashes/reloads repeatedly under yserver (works fine under Xorg) #94). Nudges reporters to check
  docs/known-issues.md first.
• Extension / feature proposal — separate form for protocol/extension
  gaps, distinct from bug reports, pointing at the Protocol scope section of
  docs/high-level-design.md so proposals that are already listed as
  non-goals get self-filtered before filing.

[joske]

This has limited value. I don't think there are any relevant X11 extensions we don't already have (GLX extensions is something else). And there are issues with the bugreport one too. Ynest is effectively dead - haven't touched it in months. Pointing users to known-issues.md is also not very practical - as I already mentioned in your other PR.