docs(troubleshooting): add support diagnostics page

docs/troubleshooting/.pages

@@ -4,3 +4,4 @@ nav:
   - LLDP Neighbors: lldp.md
   - Switch Agent: agent.md
   - ...
+  - Support Diagnostics: support_diagnostics.md

docs/troubleshooting/support_diagnostics.md

@@ -0,0 +1,63 @@
+# Support Diagnostics
+
+If the steps in the rest of this section don't resolve the issue, collect the
+following before reaching out to support. The more of this is included up
+front, the faster the issue can be localized.
+
+## What to include
+
+- Your `hhfab`/`hhfabctl` version and a summary of your topology (gateway
+  present, external peering, VPCs in use).
+- A plain description of the symptom and roughly when it started.
+- Anything that changed around that time - a config push, upgrade, reboot,
+  or node/server move - even if you're not sure it's related.
+- Which specific switches or nodes are affected, if you've already narrowed
+  it down.
+- Any diagnostics you've already collected yourself. Include the raw output
+  so we can reach our own conclusions and rule out other causes, plus your
+  own summary or theory if you have one.
+
+## Collecting a support bundle
+
+`hhfabctl` is installed on the control node as a `kubectl` plugin, so it's
+invoked as `kubectl hhfab`, not as a standalone command:
+
+```console
+core@control-1 ~ $ kubectl hhfab support dump -y
+```
+
+This produces a single timestamped `.hhs` file containing cluster resources
+(Fabricator, Agent, Connection, VPC, and related objects) and pod logs.
+Secrets are redacted, but the bundle still reflects your deployment's real
+topology and state.
+
+## Switch agent logs
+
+For issues where BGP, BFD, or interface state won't stabilize or keeps
+flapping, the support bundle's Agent CR status can look fully converged even
+while the problem is ongoing - it only reflects the most recent apply, not
+churn happening between applies. `/var/log/agent.log` on the affected
+switches is the artifact that actually shows this. It's a plain file, not a
+`kubectl` or `sonic-cli` command:
+
+```console
+admin@switch:~$ cat /var/log/agent.log
+```
+
+Include the whole file, or at least the span covering when the issue was
+observed, rather than a filtered excerpt.
+
+## Switch-level diagnostics
+
+For issues that look like a dataplane or hardware problem rather than a
+control-plane one (for example, traffic not forwarding despite correct BGP/EVPN
+state), support may also ask for a `show techsupport` capture from specific
+switches. This is a per-device SONiC command - if asked, run it only on the
+switches identified as relevant rather than the whole fabric, both to keep
+the capture a manageable size and because a comparison against a switch that
+*is* behaving correctly is often the most useful artifact.
+
+!!! note
+    Support bundles and switch dumps reflect your real deployment and should
+    be treated as confidential. Share them only through the channel your
+    support contact provides, not in a public issue or channel.