docs: add video lessons page with short explainer videos

[AztecBot]

Adds Ciara Nightingale's short Aztec explainer videos to the docs so video-first learners have a home on docs.aztec.network, plus contextual embeds on the concept pages the videos explain.

What's added

New page: Build → Resources → Video lessons (docs-developers/docs/resources/video_lessons.mdx)

Embeds all five videos with a short summary (written from their transcripts) and related-reading links into the relevant written docs:

1. What is Aztec: Explained in Under 90 Seconds → overview, foundational topics
2. One Transaction, Two Worlds: Private and Public State on Aztec → transactions, state management
3. What is Private Composability? An Aztec Explainer → call types, calling other contracts
4. How Authorization Works on Aztec → authwit concepts, aztec.nr authentication witnesses
5. Get Started on Aztec in Under 60 Seconds → getting started on a local network

Contextual embeds on related topic pages, so readers find the videos while exploring the topics they explain (each with a one-line framing sentence and a link back to the video lessons page):

Video	Embedded on
What is Aztec	Build → Foundational Topics → Aztec Overview, and Participate landing page
One Transaction, Two Worlds	Build → Foundational Topics → Transactions
What is Private Composability	Build → Foundational Topics → Call Types
How Authorization Works on Aztec	Build → Foundational Topics → Advanced → Authentication Witness, and Build → Aztec.nr → Authentication Witnesses
Get Started in Under 60 Seconds	Build → Getting Started on Local Network

Shared YouTubeEmbed component (src/components/YouTubeEmbed/index.tsx): responsive 16:9 privacy-enhanced youtube-nocookie.com iframe, replacing the copy-pasted inline iframe pattern; used by all new embeds.

Entry points for video learners

• Landing page (docs/index.mdx): new "Video Lessons" quick link in the Resources grid ("Prefer video? Learn Aztec through short explainers"). The landing page is unversioned, so this goes live on merge and deploy.
• Navbar Resources dropdown (docusaurus.config.js): "Video Lessons" entry under Other Docs, above Glossary.

Placement rationale

The Build docs already have an autogenerated "Resources" sidebar category (community calls, glossary, migration notes), so the video lessons page slots in there with no sidebar changes; it is pinned to the top of Resources via sidebar_position: 0.

Versioning

Production serves the versioned snapshots rather than docs-developers/, so the video lessons page and all contextual embeds are mirrored into developer_versioned_docs/version-v4.3.1 (mainnet, default view) and version-v5.0.0-rc.2 (testnet). Versioned sidebars are autogenerated, so no sidebar JSON changes are needed. The Participate instance is unversioned.

Validation

• yarn spellcheck passes (added "Ciara"/"Nightingale"/"Mayopg"/"VRZVO" to docs-words.txt; the last two are camelCase fragments of YouTube video IDs)
• Full yarn build succeeds; verified the built output contains all embeds on every page in the default (mainnet), testnet, and dev versions, plus the landing-page and navbar links (remaining broken-anchor/API-link warnings are pre-existing aztec_cli_reference / state_variables.md issues, unrelated to this change)

[ciaranightingale]

Adding the authwits video