docs: add SECURITY.md and GitHub issue templates

Add a security policy directing private vulnerability reports to
security@astrolabecloud.com instead of public issues, and update the
README's Security section to point at it.

Add structured issue forms under .github/ISSUE_TEMPLATE/ covering bugs,
feature requests, questions, and documentation, plus a config.yml that
disables blank issues and routes security reports and open-ended
questions to the appropriate channels. The bug template captures
fields most commonly missing from past reports (server/Nextcloud/app
versions, deployment mode, transport, MCP client).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/ISSUE_TEMPLATE/documentation.yml

@@ -0,0 +1,29 @@
+name: Documentation issue
+description: Report unclear, missing, or incorrect documentation
+title: "[Docs]: "
+labels: ["documentation"]
+body:
+  - type: input
+    id: location
+    attributes:
+      label: Where is the issue?
+      description: File path, doc URL, or section heading.
+      placeholder: "docs/authentication.md — 'Multi-User OAuth' section"
+    validations:
+      required: true
+
+  - type: textarea
+    id: problem
+    attributes:
+      label: What's wrong or missing?
+      description: Describe what was confusing, incorrect, or absent. If something didn't work as documented, include what you tried.
+    validations:
+      required: true
+
+  - type: textarea
+    id: suggestion
+    attributes:
+      label: Suggested improvement
+      description: Optional — proposed wording, missing example, or a link to a better explanation.
+    validations:
+      required: false