Remove outdated documentation folder

The docs/outdated/ folder contained deprecated content that was excluded from
the website build. This content is preserved in git history and can be
accessed through previous commits if needed.

Deleted files:
- docs/outdated/code-splitting.md
- docs/outdated/converting-from-custom-webpack-config-to-rails-webpacker-config.md
- docs/outdated/deferred-rendering.md
- docs/outdated/foreman-issues.md
- docs/outdated/rails-assets-relative-paths.md
- docs/outdated/rails-assets.md
- docs/outdated/rails3.md
- docs/outdated/tips-for-usage-with-sp6.md
- docs/outdated/upgrade-webpacker-v3-to-v4.md
- docs/outdated/webpack-v1-notes.md
- docs/outdated/webpack.md

This cleanup follows the recommendation to rely on git history rather than
maintaining hidden documentation in the main branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ihabadham