Add Docusaurus documentation site and GitHub Pages deployment

[ludgerpaehler]

Summary

• Sets up a complete Docusaurus site under docs/ with KaTeX math rendering
• Covers introduction, installation, quickstart, all four algorithms (SKVI, SAKC, LQR, SAC baselines), all four environments (LinearSystem, FluidFlow, Lorenz, DoubleWell), hyperparameter optimization pipelines, API reference, and developer contributing guide
• Adds two GitHub Actions workflows: deploy-docs.yml (deploys to GitHub Pages on push to main) and test-deploy-docs.yml (validates build on PRs to main)
• Retargets all workflows from master to main

Test plan

• test-deploy-docs.yml and Lint CI checks pass on this PR
• After merge, deploy-docs.yml deploys to GitHub Pages at https://dynamicslab.github.io/KoopmanRL/

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