Add process management docs

[sogaiu]

This PR is an attempt to address #215.

This initial version covers two approaches to working with subprocesses in Janet, i.e. os/execute and os/spawn.

There are three pages:

• a brief introduction that could be expanded to accomodate additional things if necessary in some future pr (e.g. os/posix-fork and os/posix-exec)
• a page for os/execute
• a page for os/spawn and closely related functions such as os/proc-wait and os/proc-close

FWIW, this approach of splitting across pages is similar to that taken with the fibers section which also has multiple pages.

Thanks to:

• @amano-kenji for the j3blocks work and related feedback
• @bakpakin for the tips and pointers in the various related issues
• @llmII for the repeated reviews and many discussions