docs(en): add the Using WebUI section missing from the English quick start

[louisss1016]

Description

The English and Chinese quick-start pages have drifted apart.

docs/en/GetStarted/Introduction.md and docs/zh/GetStarted/quick-start.md are the same page in two languages, but the English one stops after the CodeGenesis example list. The Chinese page continues with a 使用 WebUI section documenting the ms-agent ui entry point. English readers never see it.

This PR ports that section over, so both languages cover the same three entry points (Python API, CLI, WebUI).

Changes

One file, +14 lines, appended to docs/en/GetStarted/Introduction.md:

• a ## Using WebUI section with the pip install -U "ms-agent[webui]" / ms-agent ui commands
• the prerequisites the Chinese page states (Python 3.12+, Node.js 22.22.0+, pnpm 10.17.1)
• the Settings → Model Settings step to configure a model service before creating a project and session

Two deliberate deviations from a literal translation:

1. The link points at webui/README.md rather than webui/README_ZH.md, and uses the #first-time-setup anchor — the same target docs/en/GetStarted/Installation.md already links to, so the English pages stay consistent with each other.
2. Wording follows the existing English page ("Developers can also refer to…", the bulleted example style) instead of mirroring the Chinese sentence order.

Verification

• Diffed the two pages section by section; Using WebUI was the only section present in Chinese and absent in English. Installation, the Python example, and the CLI example already matched.
• Checked that this repo does not extract fenced blocks as runnable tests: tests/ contains no example collector (no find_examples-style helper), and .github/workflows/ has no docs job — citest.yaml runs a single unittest job. The added ```shell block is not executed.
• Format: no trailing whitespace, no smart quotes, file ends with a newline.

Notes

Not opening an issue first because this is a straight documentation-parity fix rather than a design question. Happy to split differently — for example, if the intent is for Introduction.md to stay a short overview and a separate QuickStart.md to carry the usage content, say so and I'll restructure instead.