Support more actions on comments

Comments can be assumed to be (valid) markdown (after removing /// ///! ), which we can parse and operate on. This is a list of random ideas in case someone whats to play around with this.

• fold sections based on headlines

  I.e., be able to collapse the section from # Examples to # See also in the following:

  Function that foos a bar.
  
  # Examples
  
      Some code
      Some code
      Some code
  
  # See also
  
  Baz.
  

• Resolve intra doc links

• Highlight and analyze code blocks

• Run doc tests

Start by choosing one of the listed comment actions and reviewing how rust-analyzer currently parses valid Markdown comments. Define the scope and completion criteria for that action, then identify the relevant implementation and tests before making changes.