RigidDeque/UniqueDeque: Add some top-level documentation

[lorentey]

Also, update README for the impending 1.4 release.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[Azoy]

Can we also include a link to the repo's documentation anywhere on the readme?

[lorentey]

The types the README lists are linked to the corresponding documentation page in the Swift Package Index. Do you think we need more?

[Azoy]

I’m delusional ignore me 😅