More minor formatting changes

* Convert HTML table to multimarkdown-style table to improve source-code
  readability and editability.

* Fix spacing errors in Merkle tree text diagram

* Use C-style source code highlighting for base58check code block

* (Not changed:) there's no support for JSON code highlighting. I tried
  javascript code highlighting, but it wasn't very useful.  All changes
  to JSON code blocks were reverted.

[I'll merge this formatting-only pull request in a few hours if no critical feedback is received.]