docs: split taskruns.md into how-to / reference

[vdemeester]

Context

taskruns.md is 1,190 lines mixing how-to instructions, status monitoring, debugging, and field-level reference. See Diátaxis audit for full analysis.

Part of #9740.

Proposed split

New File	Type	Source sections
tutorials/first-task.md	📘 Tutorial	(shared with tasks.md split — covers both Task and TaskRun)
how-to/run-a-task.md	🔧 How-to	"Specifying the target Task" (L88-199): inline, ref, Bundles, remote
how-to/configure-taskrun.md	🔧 How-to	Parameters (L200-369), ComputeResources (L370-408), Pod template (L409-453), Workspaces (L454-573), Sidecars (L574-687), Retries (L697-732), Timeout (L711-732), ServiceAccount (L733-742)
how-to/monitor-and-debug-taskruns.md	🔧 How-to	Status (L743-875), Monitoring (L770-875), Debugging (L936-988), Pending (L876-911), Cancelling (L912-935)
reference/taskruns.md	📖 Reference	Field spec extracted from all sections + "Delegating reconciliation" (L1151-1190)

Reading flow

📘 tutorials/first-task.md
   ├──→ 🔧 how-to/run-a-task.md
   │       └──→ 🔧 how-to/configure-taskrun.md
   │               └──→ 🔧 how-to/monitor-and-debug-taskruns.md
   ├──→ 📖 reference/taskruns.md
   └──→ 📘 tutorials/first-pipeline.md  ("Ready for multi-task workflows?")

Acceptance criteria

• Each new file has a clear single Diátaxis purpose
• Cross-links between related docs
• Old taskruns.md replaced with redirect or index
• No content lost in the split