feat: add library-effects.md intro and summary table

[magnus-madsen]

Summary

• Fills out the previously empty library-effects.md intro page
• Adds a short paragraph explaining that Flix provides built-in library effects with default handlers
• Adds a summary table linking to the four subsections: Assert, Console, Http/Https, and Process

Test plan

• mdbook build succeeds
• Verify rendered page looks correct and all links work

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