docs: introduce ixdtf.8beeeaaat.com demo site in README and godoc

[8beeeaaat]

Overview

Added links and information to introduce the demo site https://ixdtf.8beeeaaat.com/ across various documentation. The demo is an SPA based on the 8beeeaaat/ixdtf_demo repository, demonstrating interoperability by processing the same IXDTF strings using both the browser-native TC39 Temporal API and this library.

Changes

• README.md: Added a live-demo link at the beginning and created a new ## Demo section. Described it as an interoperability showcase between Temporal and Go ixdtf, mentioned multiple screens for trying out Parse / Format / FormatNano / Validate, and noted that it assumes native Temporal availability (no polyfills used).
• doc.go: Added the demo URL and a reference to the source repository in the package comments (which appear on the pkg.go.dev top page).

Verification

• gofmt -l doc.go: clean
• go build ./...: ok
• go vet ./...: ok

References

• Demo: https://ixdtf.8beeeaaat.com/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.56%. Comparing base (73a7e43) to head (0e51728).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files           8        8           
  Lines         329      329           
=======================================
  Hits          321      321           
  Misses          4        4           
  Partials        4        4           

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