DOC: Fix Multithreaded Generation example docs (gh-15367)

The old code examples did not actually run correctly and had a bug. They were
also never updated to use SeedSequence correctly. This fixes both.


* DOC, BUG: Fixes Multithreaded Generation docs

Working on #15365
The code runs now, but I get quite different runtimes than
in the example. May need some more work

* DOC: changes after PR review

* DOC: Changing input/output lines to sphinx standard

For ipython example: Adding `Out[i]` where i is the input cell number
and correcting vissual multiline inputs

* MAINT: Replace `jumped` by `SeedSequence` in multithreading snippet

After PR review, decided to replace `jumped` by `SeedSequence` as it is a more general method. Adopted the introduction to reflect this.

Also added a section on the reproducibility for different machines and number of threads + an example.

* MAINT: Changes after PR review

- Switch back to default_rng() instead of the Generator(BitGenerator)) syntax pursuant @rkern 's explanation in #15391
- small fixes

* MAINT: Delete unneccesary reference to PCG64

* MAINT: Added newline finxing codeblock error in docs

Trying to see whether this fixes circleci error

* MAINT: Clean up in imports of code snippet

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