docs: add issue forms for bugs, features, and docs

[willkg]

Adds three YAML issue forms plus a config.yml, each form applying the label it maps to (bug, enhancement, documentation — all already in use here).

Blank issues stay enabled, and config.yml carries a comment saying why. All 25 issues in this repo so far are long-form writeups with their own structure — evidence, cause, decided design, tables, code refs — and a form would be strictly worse for that. These are a floor for someone who does not know the codebase, not a shape every issue has to fit.

What each form asks for that a generic template would not:

• Bug report — the version, and a dropdown for Cloud vs. Cloud-via-the-api.atlassian.com-gateway vs. Data Center, since a bug is usually specific to one and that distinction already shapes the client. Optional field for the markdown file, noting that frontmatter matters because page_id / space / parent change what the command does. A required checkbox to confirm no token got pasted.
• Feature request — leads with the task rather than the proposed flag, and asks how they work around it today, which is what separates a convenience from a blocker.
• Documentation — splits wrong / unclear / missing, since those are three different amounts of work. For unclear it asks what the reporter concluded, because the wrong reading is the diagnostic. And it has an optional "how do you know?" scoped to docs/confluence/: those entries carry provenance by convention, so a correction about Confluence behavior needs the observation attached, or the next person establishes it from scratch.

The single contact link routes a vulnerability to the private advisory flow, which is what SECURITY.md promises.