docs: drop pre-release banner

[alcarney]

Description (e.g. "Related to ...", etc.)

No longer needed since v2 is now stable

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

Automated linters

You can run the lints that are run on CI locally with:

uv run --all-extras poe lint

[alcarney]

Thanks!