feat(tutorials): add a getting-started/quickstart page

[hhimanshu]

Summary

Adds a self-contained "Getting started" tutorial page: install @truecalc/core, run one SUM(A1, B1) example, and pointers to Tutorials / Learn / Reference. Linked from the homepage and made the first step of the beginner path, so a new visitor can go from zero to a running formula in under a minute instead of clicking through several pages first.

Closes #51

How to test

From the repo root:

npm run dev

Visit http://localhost:3000/tutorials/getting-started and confirm:

• The install snippet and the SUM(A1, B1) example render correctly.
• The "Where next" links to /tutorials, /tutorials/your-first-formula, /learn, and /reference all resolve.
• The homepage (/) and /paths/beginner now link to the new page as the first step.

Or check the executable example directly:

npm run test-docs

which runs the page's ts test block (SUM(A1, B1) against {A1: 100, B1: 200} → 300) along with every other Learn/Tutorial example.

Review

Independent review: clean. No must-fix findings. Diff touches exactly the 4 in-scope files (new content/docs/tutorials/getting-started.mdx, plus content/docs/index.mdx, content/docs/paths/beginner.mdx, content/docs/tutorials/meta.json); no generated-reference or fixture files touched.

Test plan

• npm run test-docs — 67/67 blocks passed, including the new page's example
• npm run build — static export succeeds; new page and all its outbound links resolve in the export
• npm run lint:enrichment — passes
• npm run types:check — passes

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