Update docs to reference beta 2

[jlowin]

All user-facing install commands and version badges still pointed at 3.0.0b1. This updates them to 3.0.0b2 across the README, installation guide, tasks docs, and the Mintlify nav version badge.

Historical changelog and release-notes entries for b1 are left as-is.

[coderabbitai]

Walkthrough

This PR updates version references across documentation files from "v3.0.0 (beta 1)" or "3.0.0b1" to "v3.0.0 (beta 2)" or "3.0.0b2". Changes span three documentation files: docs/docs.json (version field in dropdown), docs/getting-started/installation.mdx (installation instructions and dependency specifiers), and docs/servers/tasks.mdx (required version in installation snippet). All modifications are textual updates with no changes to logic, control flow, or error handling.

Possibly related PRs

• Prepare docs for v3.0 beta release #2954: Modifies the same documentation files (docs.json, installation.mdx, tasks.mdx) updating beta release version references
• Clarify installation for FastMCP 3.0 beta #2953: Updates how the 3.0 beta version is referenced and installed in the same documentation files
• Add missing 3.0.0 version badges and document tasks extra #2866: Modifies the same documentation pages (installation.mdx and tasks.mdx) with related version and installation content updates

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is largely incomplete and does not follow the required template, missing all contributor and review checklists with checkboxes that are mandatory per the template.	Add the required Contributors Checklist and Review Checklist sections with all checkboxes properly marked, including issue reference, workflow confirmation, testing confirmation, and self-review status.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Update docs to reference beta 2' directly and clearly summarizes the main change: updating documentation to reference the new beta 2 version instead of beta 1.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch docs/update-beta2-references

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