docs: add email client compatibility notes

[Shreyans-704]

Summary

Adds email client compatibility documentation requested in #23.

Changes

• Documents the primary target clients: Outlook, Gmail, Yahoo, and Apple Mail.
• Explains why email mode uses table-based HTML and Outlook/MSO-specific patterns.
• Documents repository-scoped compatibility caveats and clarifies that automated validation is not an inbox-client emulator matrix.
• Documents the CSP safety probe and its strict-CSP interpretation.
• Links the compatibility notes to the relevant snapshot, golden-template, browser E2E, and CSP validation files.
• Preserves the existing Litmus and Email on Acid validation information.

Validation

• pnpm test — 450 tests passed.
• git diff --check — clean.
• pnpm test:csp — currently fails because of a React CommonJS/ESM interop issue (createContext named export), not because of a CSP code-generation failure.
• No source code, tests, dependencies, or generated snapshots were changed.

Closes #23

[Copilot]

Pull request overview

Adds a new “Email Client Compatibility Notes” section to the root README to make Elements’ email-client targeting, rationale (table/MSO patterns), validation scope, and CSP gate expectations explicit for evaluators—aligning with the documentation request in #23.

Changes:

• Documented the primary targeted email clients (Outlook, Gmail, Yahoo, Apple Mail).
• Explained why <Email> renders table-based / MSO-specific markup and linked to repo evidence and validation tests.
• Documented the strict CSP probe intent and how to interpret test:csp results.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.50%. Comparing base (63d7289) to head (2810146).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
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  Coverage   95.50%   95.50%           
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  Files          31       31           
  Lines        1937     1937           
  Branches      382      382           
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  Hits         1850     1850           
  Misses         86       86           
  Partials        1        1           

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[Shreyans-704]

@ericdellacasa8
The Storybook PR preview check is currently failing at the deployment step with a 403:

Permission to unlayer/elements.git denied to github-actions[bot]

The build itself completes successfully; the failure occurs when peaceiris/actions-gh-pages@v3 attempts to push the PR preview to the gh-pages branch. Since this appears to be a workflow permission issue rather than a problem with the documentation changes, I haven't modified the workflow.