Prepare for initial release

* [ ] Re-enable lints by fixing the issues. See lints disabled at the bottom of `Cargo.toml`
* [ ] Write additional documentation.
* [ ] Write some doc tests.
* [ ] Write normal tests.
* [ ] Do we want to keep `Repeater` here or is it too specific to Velato?
* [ ] What other things should we remove?
* [ ] Peniko will soon have brush-with-alpha, so some code here can go away.
* [ ] Is there code to migrate to Kurbo or elsewhere in here? What about `SplineToPath`?
* [ ] Can Velato be made to use this or are there going to be issues with anything that blocks that? (Do a branch of Velato with a git dependency on this crate.)
* [ ] Improve the `README.md`
* [ ] Decide on a channel in Zulip and update the README.
* [ ] Compare to Motion Canvas, manim, bevy_motiongfx or others to see if we're covering essentials at this level.
* [ ] Update Vello examples to use this.
* [ ] Clarify 2D vs 3D in types (actual 3D types can come later).
* [ ] Create an issue covering what's needed for supporting CSS animation features.
* [ ] Create an issue for tracking `mint` support.
* [ ] Support tweening for a broader range of types.

Start by reading the checklist and the disabled-lint section of Cargo.toml, then inspect README.md, the existing tests, and the Vello examples. Done means the release-readiness items are completed or converted into separate decided issues, including documentation, tests, dependency and code-migration decisions, and Velato/Vello integration updates.

Written by the indexing model from the issue text.