Update the Metropolis-within-Gibbs example

[twhentschel]

Update the Metropolis-within-Gibbs example markdown notebook to be compatible with API changes, as mentioned in #551.

Included a function that takes the variance of a normal distribution as a parameter and returns a RMH kernel with a normal proposal generator. Also added minor changes to text to keep consistency with updates. I have a couple more thoughts for a few more minor edits, so I'm opening this up a draft PR (also I'm a first time contributor).

potential (nitpicky) edits:

• AlgorithmState is mentioned, but is this an actual object in the code base referring to some generic class or type? If not, I think it would be better to refer to a generic state as "algorithm state", so its not confused as something in blackjax.
• Change references of blackjax.mcmc.algorithm.init() to just init(), as the former makes it seem like there is an algorithm submodule.

Also, make test reports that 3 tests are failing related to tests/optimizers/test_pathfinder.py. But I don't think it's related to my changes.

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[junpenglao]

Thanks for your contribution! Actually the change could be a bit more simple, please see the patch that i push

[codecov]

Codecov Report

Merging #558 (5382a1b) into main (1817ee9) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #558   +/-   ##
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  Coverage   99.14%   99.14%           
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  Files          49       49           
  Lines        2099     2099           
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  Hits         2081     2081           
  Misses         18       18           

Files Changed	Coverage Δ
blackjax/mcmc/nuts.py	100.00% <100.00%> (ø)