Disallow unescaped backticks within any span-level construct other than a code span

Span-level elements that are affected by this are links (linked content,
link ref, url, title), images (alt text, ref, url, title) and
auto-links.

Within URLs, backticks are expected to be escaped to "&#96"; at other
places, backticks are expected to be escaped to "\`".
Syntax guide: Mention that backticks in link text/url/title should be escaped