Subprocess management appears under-documented

[sogaiu]

Recent activity including this issue resulted in a proposal to create a page on subprocess management for the website docs.

I looked through the existing docs and found os/spawn in a code example, but failed to locate coverage in the main text for it or os/execute. If I've missed anything relevant, I hope someone will point such things out (^^;

The aforementioned issue appears to contain relevant bits of information regarding the use of subprocesses in Janet so hopefully we can capture a useful portion of it in some manner.

Also of note is this PR. Whatever the fate of that, there may still be bits from it that are useful for our purposes here.

[sogaiu]

For any interested parties, some related work is taking place here.

[sogaiu]

Closing as #218 was merged.