Fix Commonmark issues

[hdevalence]

@cathieyun pointed out that some formulas are broken; we found that it seems to be because the markdown writes \\\\ to get \\ in the HTML for a latex line-break, but Commonmark sometimes emits a <br>, which causes the formula to be broken into two text spans and fail to parse as math. Curiously, this does not happen on my local machine, so I temporarily deployed from my local machine to the hosting service, bypassing the deploy pipeline, but it will break again the next time the repo is updated.

[cathieyun]

In the meantime, you can view PDFs of the correctly-rendered notes here:
https://drive.google.com/drive/folders/12JU_0lhVzcgiF6loXgMRCwUZ0MOZjzCw?usp=sharing

Or you can render the notes yourself:

1. clone the bulletproofs repo
2. inside the bulletproofs folder, run:
   make doc (if you want the external library API notes) or
   make doc-internal (if you want the internal notes)
3. in a browser, open:
   <bulletproofs directory>/target/doc/bulletproofs/notes/index.html to get the "notes overview" page.