[Docs] Finish remaining examples revamp follow-ups

[peterschmidt85]

Follow-up items remaining from #3798.

• Finish moving to inline examples only
• Get rid of examples generation: move all examples from /examples to docs and remove the corresponding hook
• Update TRL examples under Single-node training
• Update AMD example
• Update TRL examples under Distributed training
• Update TPU example

[peterschmidt85]

PR #3859 addresses the structural items:

• Finish moving to inline examples only — 20 navigated examples now have all source code inline; examples/<...>/<n>.dstack.yml deleted
• Get rid of examples generation — example materialization hook removed, scripts/docs/gen_examples.py deleted, files moved to docs/docs/examples/ (URLs now /docs/examples/... with redirects from old paths)
• (Bonus) Single-node training and distributed training merged into a single "Training" section; TRL and Axolotl pages have ## Single-node training and ## Distributed training H2 sections

Content updates still pending — left as follow-ups so this PR stays focused on the structural cleanup:

• Update TRL example content
• Update AMD example content
• Update TPU example content