Handbook: Add a small chapter about doas as an alternative to sudo

[gbe]

Summary

Handbook: Add a small chapter about doas as an alternative to sudo

doas is a viable alternative to sudo, so it is worth to be
mentioned in our handbook.

[gbe]

Anyone up for a review?

[juan.molina_club.fr]

Typos

documentation/content/en/books/handbook/security/_index.adoc

2134

s/specifig/specific

2136

s/simpliest/simplest

2142

s/privilages/privileges

[0mp]

Should we mention that doas cannot cache passwords on FreeBSD?

Otherwise, seems good!

documentation/content/en/books/handbook/security/_index.adoc

2130

Should put a comma here?