Move information from Void Web site to Handbook

[flexibeast]

As described in void-linux/void-linux.github.io#90, we need to make sure the content in the usage/runit, usage/XBPS and FAQ pages on the Void Web site is instead covered by the Handbook.

• usage/runit
  /index: Point to Handbook re. runit, remove runit/index.markdown. void-linux.github.io#104
• usage/XBPS
  Ensure relevant content from usage/xbps/ on Web site is covered by Handbook #358, index.html: Point to Handbook for XBPS info, capitalise 'XBPS'. void-linux.github.io#107
• FAQ
  Ensure relevant content from Web site FAQ is covered by Handbook #346, faq/index: Remove. void-linux.github.io#105

[ericonr]

Regarding runit:

It mentions all the runit related utilities, but they are already mentioned in runit's man pages, and their function is explained in the relevant sections of the handbook, if needed.

It has an example for how to switch the runsvdir, using

runsvchdir my_runlevel

which I have no idea if it's recommended or not.

[flexibeast]

Blocked on #346.

[flexibeast]

@ericonr: i think the reference to the runsvchdir(8) man page in the "Services and Daemons - runit" section is probably sufficient. More generally, it seems to me the Handbook adequately covers the contents of usage/runit; if you agree, i'll open a PR in void-linux/void-linux.github.io to modify the landing page to point to the above Handbook section, and to remove usage/runit.

[flexibeast]

Blocked on #358.

[flexibeast]

With #358 now closed, this issue can be closed.