Clarify rules for code blocks, including spaces
above and below, and to use tilde when code blocks
contain triple backticks

Explain indentation more carefully, and expand examples

Let's use disqus_identifier since this is a more
descriptive name. We will need redirect_from for
another purpose anyway (Nanoc redirects).

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for most docs in
/development, and cleans up other whitespace issues too

Following the single docs codebase change, all internal links
should be relative. Also cleans up one table