doc/requests: add small introduction

This should make the API doc more inviting.
doc/requests: expand a little on POST
doc/requests: expand on why one would issue GET requests
doc/requests: stress where to POST a job description

Everett Toews asked about this on IRC.
doc/requests: group POST and GET documentation

Before, the documentation for POST and GET requests were interleaved.
doc/requests: split paragraphs into real paragraphs

The document had many long lines with single newlines here and there.
I've now separated what looks like differnet paragraphs into real
Markdown paragraphs (separated by two newlines) and rewrapped the
lines to it's easy ot see what belongs where.
doc/requests: add quotes consistently
doc/requests: add internal links to make navigation easier
doc/requests: add link to job description docs
doc/requests: expand a little on what a ZeroVM image is
doc/requests: drop unnecessary quotes

In this context, "root" is not a literal.
doc/requests: make it explicit that the POST data is the script
doc/requests: rephase what a shebang line is
doc/request: add real link to Configuration.md
Merge pull request #73 from mgeisler/requests-prose

Add more prose to requests API docs