fix(docs): add missing static files to build

[villebro]

SUMMARY

Currently the built docs assets are missing two critical files required for deploying the Superset www site. This adds an additional script copy-www-site-files to the docs package.json that is executed before the build is deployed to the apache/superset-site repo.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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