Add a couple more demos for our live version with MDX

[cmdcolin]

This creates a link to a couple of our preloaded config demos (human with sample data, yeast dotplot, grape vs peach dotplot, breakpoint split view demo, volvox)

Also adds BOG2021, and moves the cancer demo to an embedded demo section

https://jbrowse.org/jb2/demos/

The linear synteny demo is not present due to #1769

[codecov]

Codecov Report

Merging #1791 (4dfb374) into master (afdd9e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1791   +/-   ##
=======================================
  Coverage   58.66%   58.66%           
=======================================
  Files         457      457           
  Lines       21193    21193           
  Branches     5016     5016           
=======================================
  Hits        12432    12432           
  Misses       8451     8451           
  Partials      310      310           

---

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 afdd9e4...4dfb374. Read the comment docs.