Building -> Optimizing docs

We should add a page to explain new optimization concepts in Ionic 4:

- Build size !== App size
- The "loader" under the hood and how it's different from a bundle, even though there are still bundles
- All the SVGs! 😨
- How to use DevTools to inspect time to first paint, app size, general performance tuning

Use the issue’s requested topics as the content outline: build size versus app size, the loader and bundles, SVGs, and DevTools performance inspection. Review the existing Ionic 4 documentation structure first, and consider the page complete when it explains each topic and covers time to first paint, app size, and general performance tuning.

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