Added multiple data example to plot_forest

[Rishabh261998]

Description

Fixed #1706

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1761 (4fc068b) into main (928f8f4) will decrease coverage by 3.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1761      +/-   ##
==========================================
- Coverage   91.18%   87.27%   -3.92%     
==========================================
  Files         115      115              
  Lines       12248    12248              
==========================================
- Hits        11168    10689     -479     
- Misses       1080     1559     +479     

Impacted Files	Coverage Δ
arviz/plots/forestplot.py	94.59% <ø> (ø)
arviz/data/io_pymc3_3x.py	18.18% <0.00%> (-72.06%)	⬇️
arviz/data/io_pystan.py	62.12% <0.00%> (-33.96%)	⬇️
arviz/data/io_cmdstanpy.py	70.68% <0.00%> (-24.76%)	⬇️
arviz/data/io_emcee.py	93.38% <0.00%> (-4.96%)	⬇️
arviz/utils.py	88.81% <0.00%> (-0.33%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 928f8f4...4fc068b. Read the comment docs.