Add Chapter 2 contents (#45)

Squashing a few commits from edx-issue45 together into a single commit:

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Populate content from Chapter 2 of edX course.

Chapter 2 is Introduction to Hyperledger.

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Incorporate feedback to PR #62 for Issue #45

For images, add Licensed under CC By 4.0 with link.

For videos with "some rights reserved", remove it.

Replace a., b. with 1., 2. to get indented sub-#s.

Update Hyperledger Burrow link.

Remove source of Hyperledger Cello image because of CC By 4.0.

Signed-off-by: nathalie-ckc <nathalie.ckc.123@gmail.com>

Author: nathalie-ckc