docs: bump version to 0.2.1

[lbliii]

Summary

• Bump docs version from 0.2.0 to 0.2.1 to match the 0.2.1 release (docs/conf.py release, docs/project.json version).
• Add 0.2.0 as a previous version in docs/versions1.json version switcher (it was missing); latest now represents 0.2.1.

Test plan

• Verify the version switcher renders 0.2.0 alongside 0.1.x entries in the built docs.

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