add sampling benchmark

[mileslucas]

• add sampling benchmark
• add benchmark section to docs

I've noticed something concerning- in the evidence estimation we have a pathological bias. This is similar to the bias shown by MultiNest outlined in the JAXNS paper.

I'll be looking into this in a subsequent PR.

[codecov]

Codecov Report

Merging #79 (6eeeec7) into main (1c86d76) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #79   +/-   ##
=======================================
  Coverage   88.62%   88.62%           
=======================================
  Files          13       13           
  Lines         554      554           
=======================================
  Hits          491      491           
  Misses         63       63           

Impacted Files	Coverage Δ
src/bounds/ellipsoid.jl	85.39% <ø> (ø)
src/bounds/multiellipsoid.jl	78.18% <ø> (ø)
src/proposals/Proposals.jl	92.98% <ø> (ø)

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