docs: update Flask-Babel links

[haocheng6]

SUMMARY

• This PR updates Flask-Babel links in the documentation site because the old links do not work now.
• This PR also updates the Babel link in the contributing guide to make it consistent with the documentation site.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Build the documentation site locally and verify that the new links work as expected.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[github-actions]

Congrats on making your first PR and thank you for contributing to Superset! 🎉 ❤️
We hope to see you in our Slack community too!

[rusackas]

Thanks for the updates, @haocheng6! We always love it when people help touch up the documentation. If you have further interest in this effort, ping us here or on Slack!