docs: use --prerelease=explicit for install instructions

[zzstoatzz]

Using --prerelease=allow when installing fastmcp can pull in prerelease versions of dependencies like httpx. httpx 1.0.dev* removes TransportError which breaks httpx-sse, which breaks mcp imports.

--prerelease=explicit only allows prereleases for packages explicitly requested with a prerelease specifier (like fastmcp>=3.0a0), keeping dependencies on stable versions.

# before (could install httpx==1.0.dev3)
uv add fastmcp --prerelease=allow

# after (installs httpx==0.28.1 stable)
uv add "fastmcp>=3.0a0" --prerelease=explicit

🤖 Generated with Claude Code

[jlowin]

pip install "fastmcp>=3.0.0b1" --pre is still problematic (gets the bad httpx version) -- but it turns out pip install 'fastmcp>=3.0.0b1' is fine! Going to close this and adopt that in #2975. Thanks!