verify that a manpage doc is valid

[xcoulon]

if a document has a doctype=manpage attribute, we should verify that it conforms to the following rules:

Document Header

A man page document header is mandatory. The title line contains the man page name followed immediately by the manual section number in round brackets. The title name should not contain white space. The manual section number is a single digit optionally followed by a single character.
The NAME Section

The first man page section is mandatory, must be titled “NAME” and must contain a single paragraph (usually a single line) consisting of a list of one or more comma separated command name(s) separated from the command purpose by a dash character. The dash must have at least one white space character on either side.
The SYNOPSIS Section

The second man page section is mandatory and must be titled “SYNOPSIS”.

Subsequent sections are optional, but typical sections include “SEE ALSO”, “BUGS REPORTS”, “AUTHORS” and “COPYRIGHT”.

See https://asciidoctor.org/docs/user-manual/#man-pages

If the document does not compy, it should be rendered as a regular article (ie, default rendering templates) and an ERROR should be raised in the console.