Make mandocs.css reproducibile

[avivace]

We modify a bit the mandocs.css distributed upstream with the mandoc package:

• to scope some of the rules inside the manual-text classes, so they don't get applied to all the website
• to remove some of the properties (e.g. font-face, width) so the website themes one get used

We should script this so we can pull upstream a recreate our version without a human intervention