docs: update README and regenerate documentation

* Improve README with clearer introduction and feature overview
* Add “What Makes RcppAlgos Unique” section highlighting specialized algorithms
* Mention prime utilities and acknowledge Kim Walisch’s work
* Refine usage examples and iterator demonstration
* Regenerate vignette outputs and documentation under doc/
* Update links to new articles for partitions, compositions, and constraints

This commit refreshes the project documentation to better highlight
RcppAlgos capabilities and align the README with the pkgdown site.

Author: jwood000