Adding html directory for website_B

[ashwinimarathe]

• We have introduced a new section called Learning paths just below Class Schedule in the sidebar
• We created an a new directory called html_B at ./docs/html/ for the change
• Since all the webpages had the code for the sidebar, we had to touch all the HTML files and make the required change

[nickeubank]

Three notes:

• There's an index.html file in docs that is the "master" file for github pages. Right now it's designed to redirect to docs/html/index.html. Not sure how that affects traffic routing.
• You'll need to delete all HTML pages and rebuilt to get the menu to rebuild, otherwise the "learning path" will appear and disappear:

• You may want to add a note about the "learning paths" at the bottom of all exercises (say, "want more exercises? Check out a list of all available exercises here!") to improve findability.

[ashwinimarathe]

• The index.html file in docs will redirect to /html/index.html, so we plan to use html/index.html as the original page and html_b/index.html as the variant, so that anytime someone hits html/index.html half the time they will be redirected to html_b/index.html.
• Will do that and push again

[nickeubank]

Great!

[nickeubank]

Looks good! I'll merge now so you can start playing with Optimize, but feel free to submit more PRs if you want more edits.

[ashwinimarathe]

• Updated the sidebar for exercise pages to display the Learning Tracks link