[Doc][Last/N] Improve all pooling task | Refactor pooling-related documentation

[noooop]

Improve all pooling task

• [Frontend][Doc][5/N] Improve all pooling task | Polish encode (pooling) api & Document. #25524
• [Model][-/N] Improve all pooling task | Support chunked prefill with ALL pooling #27145
• [Model][6/N] Improve all pooling task | Improve enable chunked_prefill & prefix_caching logic. #26623
• [Model][7/N] Improve all pooling task | Deprecation as_reward_model. Extract hidden states prefer using new multi-vector retrieval API #26686
• [Doc][Last/N] Improve all pooling task | Refactor pooling-related documentation #27963

These PRs are mostly conflicting with each other, so combining them into a series would better inform reviewers about what happened. And what else needs to be done after that?

Purpose

Refactor pooling-related documentation

1. With the addition of new features, it's not just about Pooling Models anymore, but rather more focused on pooling task and pooling API. Should we change the title to something more appropriate?

2. Find a way to put this figure into the document

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--27963.org.readthedocs.build/en/27963/