Clarify installation for FastMCP 3.0 beta

[jlowin]

The v3 docs show install commands that would install the stable 2.x release, not the 3.0 beta. Since users reading the 3.0 docs want 3.0, the default instructions should include the pre-release flags.

Changes:

• Add --pre (pip) and --prerelease=allow (uv) to all install commands
• Add a note explaining that 3.0 is in beta and requires explicit opt-in
• Add a tip to the Background Tasks doc pointing to installation instructions

[coderabbitai]

Walkthrough

This pull request updates installation documentation for FastMCP 3.0, which is currently in beta and requires explicit prerelease handling. The changes modify installation instructions across multiple package managers (uv add, uv pip install, and pip install) to include appropriate prerelease flags. Additionally, the documentation clarifies optional dependency installation, specifically for the background tasks extra, and adds informational notes about feature requirements in the tasks documentation.

Possibly related PRs

• Add missing 3.0.0 version badges and document tasks extra #2866: Modifies the same documentation pages (installation and tasks) to document the tasks extra feature and its installation requirements.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description provides clear context about the problem (install commands defaulting to 2.x), explains the specific changes made (prerelease flags, beta note, background tasks tip), but lacks the required checklist items from the template.	Complete the Contributors Checklist and Review Checklist by checking off the relevant items and confirming issue closure, testing, and self-review status.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main objective: updating documentation to clarify installation steps for the FastMCP 3.0 beta release with prerelease flags.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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