Doccords for arms

[drbeefsupreme]

"arm docs"
Docs written above the arm, e.g.

:>  This is an arm doc
++  foo

"product docs"
Docs written after the name of the arm, e.g.

++  foo
:>  This is a product doc

"default arm in core docs"
If an arm builds a core that has an arm named $, then the arm doc of that arm, e.g.

++  foo
  |%
  :>  This is docs on a default arm
  ++  $  ~

These all currently mostly work, but some edge cases fail. Lots of product cases have yet to be written, but there are some parser issues when multiple cases are combined.

[drbeefsupreme]

Not worth addressing edge cases until #5770 is resolved