Add diagnostics platform, config-flow gap warning, issue template

Completes the device-capability-diagnostics spec:

- diagnostics.py: the standard HA diagnostics hook, returning device type,
  one_ui_version, unbound hrefs, and the redacted raw resource tree, plus
  integration and smartthings-local version numbers. This is what the
  Repairs issue (added in the previous commit) points users at, and what
  they attach to a device-support issue.
- config_flow.py: _probe_and_validate now also reports oneUiVersion and
  whether the device type is recognized. Recognized types are unaffected;
  an unrecognized type shows a new confirm_unknown_type step explaining
  that only common capabilities will be available before creating the
  entry, so expectations are set at setup time rather than only after the
  fact via Repairs.
- .github/ISSUE_TEMPLATE/device-support.yml: structured template for
  filing a capability gap, linked from both the Repairs issue and the
  config-flow confirmation step.
- README: short section pointing at this whole mechanism.

.github/ISSUE_TEMPLATE/device-support.yml

@@ -0,0 +1,31 @@
+name: Device support / capability gap
+description: Report an appliance type that wasn't recognized, or one that's missing some capabilities
+labels: ["device-support"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for helping expand device coverage. The most useful thing you can attach here is a diagnostics
+        download: Settings > Devices & Services > this device > the menu (top right) > Download diagnostics.
+        It's already redacted of account/network identifiers before it's generated.
+  - type: input
+    id: model
+    attributes:
+      label: Appliance model
+      description: The model number or product name, if known (e.g. from a label on the appliance or its manual)
+    validations:
+      required: false
+  - type: textarea
+    id: description
+    attributes:
+      label: What's missing or wrong
+      description: What entities are missing, or what behavior didn't match what you expected
+    validations:
+      required: true
+  - type: textarea
+    id: diagnostics
+    attributes:
+      label: Diagnostics file
+      description: Drag and drop the downloaded diagnostics JSON file here
+    validations:
+      required: false