Render a non-empty plot in Mario-RL tutorial

[kiersten-stokes]

Fixes #1607

Description

This PR makes a few changes in order to render a non-empty plot in the doc HTML.

• Ups the number of episodes run so that there are enough data points to plot (w/ 10 episodes, only one data point would be available due to a save every 20)
• Clears the plot at the beginning of the loop that plots all 4 metrics; this ensure the plot is not cleared on the final iteration and results in something being displayed
• Adds a legend to the plot so that the rendered image contains some context and isn't just a random straight line

New rendering:

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

cc @vmoens @nairbv @svekars @carljparker

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[kiersten-stokes]

Interestingly, in the deploy preview I'm not getting any image render (see screenshot below). But I suppose this still fixes the original issue

[svekars]

Will need to wait for the manager worker to finish and check in the build artifacts. Netlify only runs the simple make html-noplot without actually running the code and generating outputs. It's a quick preview for .rst/HTML only fixes.

[svekars]

Looks good: https://output.circle-artifacts.com/output/job/0090a54a-4baa-444f-9d18-cb3798cc6804/artifacts/0/tutorials/intermediate/mario_rl_tutorial.html