mdocml: fix readability issues in void-man-cgi.

[mjyut]

Fix: #36557

Testing the changes

• I tested the changes in this PR: briefly
  I've confirmed the changes in CSS.
  However, I have not tested it in the same environment as https://man.voidlinux.org/ .

Local build testing

• I built this PR locally for my native architecture, (x86_64-glibc)

[leahneukirchen]

Thanks, LGTM.

I'd replace the #mandoc line with

.manual-text { max-width: 65em; padding: 0 1em; }