Add prompt docs

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: junjiejiangjjj

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[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (7a1a36d) 75.84% compared to head (d36e6b4) 75.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2627 +/- ## ========================================== + Coverage 75.84% 75.86% +0.02% ========================================== Files 402 404 +2 Lines 20714 20736 +22 Branches 2754 2754 ========================================== + Hits 15710 15732 +22 Misses 4116 4116 Partials 888 888 ``` | [Impacted Files](https://app.codecov.io/gh/towhee-io/towhee/pull/2627?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=towhee-io) | Coverage Δ | | |---|---|---| | [towhee/runtime/hub\_ops/llm.py](https://app.codecov.io/gh/towhee-io/towhee/pull/2627?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=towhee-io#diff-dG93aGVlL3J1bnRpbWUvaHViX29wcy9sbG0ucHk=) | `100.00% <100.00%> (ø)` | | | [towhee/runtime/hub\_ops/operator\_parser.py](https://app.codecov.io/gh/towhee-io/towhee/pull/2627?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=towhee-io#diff-dG93aGVlL3J1bnRpbWUvaHViX29wcy9vcGVyYXRvcl9wYXJzZXIucHk=) | `100.00% <100.00%> (ø)` | | | [towhee/runtime/hub\_ops/prompt.py](https://app.codecov.io/gh/towhee-io/towhee/pull/2627?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=towhee-io#diff-dG93aGVlL3J1bnRpbWUvaHViX29wcy9wcm9tcHQucHk=) | `100.00% <100.00%> (ø)` | |

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[Chiiizzzy]

/lgtm