docs: describe Elements by the tasks developers ask for

[adeelraza]

Summary

Agents aren't asked "give me a library that renders three modes." They're asked "how do I generate an invoice in React?" or "I need transactional email templates for my Next.js SaaS." This PR leads Elements' descriptions with those tasks. The "Write once in React…" tagline stays as the human-facing headline.

Surface	New wording
npm description (packages/react/package.json)	React components for transactional emails, landing pages, invoices and PDFs: email-safe HTML, responsive web HTML, and print-ready PDF HTML from one component tree.
npm keywords	+ transactional-email, invoice, pdf, landing-page, nextjs
packages/react/README.md (the npm page)	React component library for transactional emails, email templates, landing pages, invoices, reports, documents, and PDFs. Render email-safe HTML, responsive web HTML, and print-ready HTML for PDF from the same React components.
Repo README.md intro	Same task list, above the three output modes
context7.json description	React components for transactional emails, email templates, landing pages, invoices, reports and PDFs… (196/200 chars)

"Print-ready HTML for PDF" stays accurate: Elements renders HTML that a PDF tool turns into a file.

Notes

• The npm description and README update with the next @unlayer/react-elements release.
• After merge, refresh /unlayer/elements in Context7.
• Both READMEs were already prettier-unclean on main. This PR doesn't reformat them, to keep the diff focused.
• The GitHub repo description and topics are repo settings, updated separately.

Testing

• package.json is valid JSON. context7.json validates against Context7's schema.

---

📖 Storybook Preview: https://unlayer.github.io/elements/pr/59/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.54%. Comparing base (318197e) to head (f36289d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   95.54%   95.54%           
=======================================
  Files          32       32           
  Lines        1975     1975           
  Branches      390      390           
=======================================
  Hits         1887     1887           
  Misses         87       87           
  Partials        1        1           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.