[Doc] document pip wheel install path and version matching

[Zymonody7]

Add a quickstart section for installing vllm-metax pre-built wheels from the MetaX index: maca/torch/mcoplib matching table, mcoplib split since v0.14, --no-deps install for upstream vllm and common pitfalls verified on Gitee.AI C500 images (maca 3.2.1 / 3.3.0.15 / 3.7.1.5).

Purpose

The quickstart only documents the docker path, while pre-built wheels are
published to repos.metax-tech.com but undocumented (index URL, maca/torch
matching rules, mcoplib split since v0.14, how to install upstream vllm
without the cuda dependency set).

This adds an "Install via pip (pre-built wheels)" section + a wheel matching
table + 4 pitfalls, all hit and verified on Gitee.AI C500 instances
(MACA 3.2.1 / 3.3.0.15 / 3.7.1.5, torch 2.6/2.8, vllm-metax 0.13/0.17,
mcoplib 0.3.1/0.4.5).

Test Plan

Docs-only change; commands in the section copied verbatim from a working
install session on a C500 cloud instance.

Test Result

vllm 0.13 + vllm-metax 0.13 (bundled kernels) and mcoplib 0.4.5+maca3.7
import & run topk ops on MACA 3.3.0.15 and 3.7.1.5 respectively.

(Optional) Documentation Update

This PR is the documentation update.

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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.

[gemini-code-assist]

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