doc updates

[richfitz]

• Description contains reference to wrong rng (or rather, to just one)
• README contains now-incorrect vignette("cuda") (may be elsewhere)
• no references to the two new rng vignettes
• rng vignette add polar to benchmarks, add links to algorithms doc
• rnguse.cpp package might be stale? who tests this?
• not sure that I have the right syntax highlighting for c++ blocks, switch to c++ and it should look better
• off-by-ones in the ziggurat docs
• the polar and BM methods not really explained
• "Running this same code in series will give the same results" (dust.Rmd) but we did not set the seed so results are different!
• better pkgdown structure following https://github.com/r-lib/rlang/blob/main/_pkgdown.yml
• reference docs for dust_rng's jump method is incorrect in the length of jump
• errors in distributed doc ("## Error in matrix(r2$state(), 32): object 'r2' not found"), and also not running with errors-are-errors
• names on set_index is not documented (in run, simulate, set_index) - see Document index naming mcstate#158