Fix several errors in the manual

I fixed several errors in the manual:

* bd67bb8
  - the output of `unique_by(length)` example was not sorted by length.
  - fix typo: trailing ']'.
  - the output of `recurse` example were missing `.` object itself.

* fdbc91e
  - programs, inputs and outputs were not HTML-escaped, causing named-capture in regexes to disappear in html. (this also fixes #589)

* b4a9ea5
 - fix yaml indentation around the regex section because the generated html were corrupted.

* bd67bb8
 - fix markup, unmatched backquotes.
 - jq output not being represented as a list in yaml were causing "none"s in generated html.

They are commited separately because the indentation fix would have obscured other changes.
Please squash them if it is more convenient to merge.