docs(dev): collapse per-language extension guides into <details> switches

[noron12234]

Closes #1994.

Problem

The extension development guide (docs/development/how_to_develop_with_ext.md and its .cn.md counterpart) tiled all four language sections (C++, Go, Python, Node.js) on one page. A reader looking only for Python still had to scroll past ~200 lines of C++ and ~170 lines of Go before reaching the guide they wanted. The Chinese version is the same shape and had the same problem.

The issue asks for switch-like behavior instead of tiling.

Fix

Wrap each language section in a collapsible <details><summary> block. Readers now see a compact list of four language guides and expand the one they need.

## Language-specific Development Guides

Click a language below to expand its full guide.

<details>
<summary><strong>C++ Extension Development</strong></summary>

...C++ content unchanged...

</details>

<details>
<summary><strong>Go Extension Development</strong></summary>

...Go content unchanged...

</details>

...Python...
...Node.js...

Same shape in both how_to_develop_with_ext.md and how_to_develop_with_ext.cn.md.

Why this shape

• Matches an existing convention — docs/getting-started/websocket-voice-assistant-quick-start.md already uses <details><summary><strong> for its Troubleshooting section (five blocks). Reusing that pattern rather than inventing something.
• Framework-neutral — plain HTML <details> renders on GitHub and on any Fumadocs / MDX / static-site renderer without additional imports. No import { Tabs } shim required.
• Zero content edits — the inner content of each language section (including the ### subheadings that generate TOC anchors) is untouched. Diff is boundaries only.

Verification

$ grep -c '^<details>' docs/development/how_to_develop_with_ext.md    #  4
$ grep -c '^</details>' docs/development/how_to_develop_with_ext.md   #  4
$ grep -c '^<details>' docs/development/how_to_develop_with_ext.cn.md #  4
$ grep -c '^</details>' docs/development/how_to_develop_with_ext.cn.md #  4

Both files preserve their inner ### subheading structure (Create Project, Run Tests, etc.) so any TOC generator or deep-link that pointed at those anchors still resolves.

Files changed

• docs/development/how_to_develop_with_ext.md (+24 −7)
• docs/development/how_to_develop_with_ext.cn.md (+24 −7)

Alternatives considered

• MDX <Tabs> component — would need to convert the file to .mdx and add an import; larger blast radius and framework-specific. Happy to switch if the maintainers prefer tabs and can point me at the right component import path.
• Splitting into four files (per language) — cleaner but changes URLs and breaks any external links currently pointing at #c-extension-development etc.