### Checklist Before Starting
- [x] Searched for similar PR(s).
- [x] Checked PR Title format
- In format of: [modules] type: Title
- modules are in `fsdp, megatron, sglang, vllm, rollout, trainer, ci,
training_utils, recipe, hardware, deployment, ray, worker,
single_controller, misc, perf, model, algo, env, tool, ckpt, doc, data`
- type is in `feat, fix, refactor, chore, test`
- can involve multiple modules, seperated by `,` or space, like
`[megatron, fsdp, doc] feat: xxx`
### What does this PR do?
This PR fixes the broken link for DAPO branch and add some details to
the doc.
### Checklist Before Submitting
- [x] Read the [Contribute
Guide](https://github.com/volcengine/verl?tab=readme-ov-file#contribution-guide).
- [x] Apply [pre-commit
checks](https://github.com/volcengine/verl?tab=readme-ov-file#code-linting-and-formatting).
- [x] Add `[BREAKING]` to the PR title `description` if it breaks any
API.
- [x] Update the documentation about your changes in the
[docs](https://github.com/volcengine/verl/tree/main/docs).
- [x] New CI unit test(s) are added to cover the code path.
- [x] Rely on existing unit tests on CI that cover the code path.
