fix(docs): normalize sidebar path separators for Windows

[sidsri14]

Fixes #2402: building the docs sidebar on Windows fails with Cannot read properties of undefined (reading 'toLowerCase') because path.relative returns backslash-separated paths on Windows, so url.split('/')[1] (the category) is undefined.

What changed

• packages/docs/sidebars.js: normalize the doc-relative path to forward slashes (\\ → /) before category extraction and doc-ID construction, via a small relativeDocUrl helper. The generated doc IDs remain forward-slash paths on all platforms.
• Added packages/docs/sidebars.test.js (Node's built-in node:test runner, no new dependencies) and a test script for @temporalio/docs. The tests cover forward-slash, Windows-style, and multi-dot basename inputs. CI's pnpm run test runs on windows-x64 too, so this is a real regression test for the affected platform.

Verification

• node --test packages/docs/sidebars.test.js → 3/3 pass.
• prettier --check clean on all changed files.
• Full docusaurus build requires a dependency install and POSIX shell for the build-docs env assignments, so it wasn't run locally; the bug reproduces purely in sidebar generation, which the unit tests exercise.

AI disclosure

Code authored with the assistance of an AI coding agent.