Markdown lint warnings

Review of !1600 (merged) has highlighted some minor markdown formatting degradations, flagged by the code quality metrics, that have snuck into the documentation at various points. This is a master issue to clean them up.

As identified by the report:

• "Lists should be surrounded by blank lines"
  • doc/bitcoin-conf.md:14
  • doc/bitcoin-conf.md:26
  • doc/bitcoin-conf.md:32
  • doc/developer-notes.md:520
  • contrib/bench/README.md:34
• "Inconsistent indentation for list items at the same level"
  • doc/developer-notes.md:512
• "Unordered list indentation"
  • doc/developer-notes.md:529
• "Fenced code blocks should be surrounded by blank lines"
  • doc/developer-notes.md:515
  • doc/developer-notes.md:519
• "Bare URL used"
  • src/test/data/CHIPs/README.md
• "First line in file should be a top level header"
  • src/test/data/CHIPs/README.md

Out of scope

This issue considers line length warnings out of scope, as there are some legitimate cases for very long lines in the docs, ignoring the warning is not a good idea as it is valid elsewhere, but selectively silencing it for some files does not appear to be possible.

Ditto for inline HTML.