feat(docs-portal): sticky nav bar with active section highlighting

[adepoju2006]

Close #89

Summary

Adds a sticky in-page section navigation bar with scroll-spy active highlighting to the docs portal.

This also scaffolds the docs-portal/ Docusaurus site itself — the api-docs-portal.yml workflow already expected a docs-portal/ directory (it runs npm run build there), but the directory did not exist in the repo. This PR creates it and implements the feature inside it.

What's included

• useScrollSpy.ts — an IntersectionObserver-based hook that returns the id of the section currently in view. SSR-safe (all DOM access inside useEffect); when multiple sections are visible at once, the topmost in document order wins.
• ScrollSpyNav/index.tsx — the sticky <nav> component: active-link highlighting, click → smooth scroll, URL hash sync, and focus moved to the target section for keyboard / screen-reader users.
• styles.module.css — position: sticky pinned below the Docusaurus navbar at every breakpoint (horizontal scroll on mobile), with WCAG-AA color states.
• src/pages/index.tsx — demo home page wiring the nav to six on-page sections so the behavior is visible and testable.
• Minimal Docusaurus scaffold: docusaurus.config.ts, sidebars.ts, tsconfig.json, babel.config.js, src/css/custom.css, docs/intro.md, logo, README.

Acceptance criteria

Criterion	Implementation
Fixed at top on all screen sizes	position: sticky; top: var(--ifm-navbar-height); list scrolls horizontally below 600px
Active link highlighted on scroll	useScrollSpy + IntersectionObserver apply .linkActive + aria-current
Smooth scroll on link click	scrollIntoView({ behavior: 'smooth' }), honors prefers-reduced-motion
WCAG 2.1 AA contrast (active + inactive)	Inactive #44474e / #d0d3d9 (~8.9–12:1), active white on #1d4ed8 (~6.6:1); active also signalled by weight + underline, not color alone

Accessibility notes

• Active state is conveyed by color and weight and an underline bar — never color alone.
• aria-current="true" on the active link; aria-label on the nav landmark.
• Honors prefers-reduced-motion (both the JS smooth-scroll and CSS scroll-behavior).
• Focus is moved to the target section on click for keyboard/SR users.

Verification

• npm install
• npm run build (clean, no warnings — generates build/index.html, matching what CI uploads)
• npm run typecheck

How to test locally

cd docs-portal
npm install
npm start   # then scroll the home page and click the nav links

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