Overhaul deployer documentation

[joshuaauerbachwatson]

issue #54.

The "overhaul" is complete for now and ready for review and merge.

More adjustments and minor fixes are certainly possible but I don't think they are necessary immediately.

[joshuaauerbachwatson]

I have marked this reviewable (and I think it might be merged) despite being, in some ways incomplete. The current status:

• The new specification document is written. It might need some revisions but it's hard to say what's needed without a pass around some other parts of the doc
• Other parts of the documentation have been touched in only a few places (this includes the sidebar, so the new document is reachable). The next step ought to be to more thoroughly revise other parts of the documentation to reference and supplement the new document more gracefully.
• Most notably, the old configuration section still exists and is still referred to. Thus, many configuration options are documented twice. The more recent discussion in the spec is based on a recent review of the code and more likely to be accurate but there may still be some useful things in the old document. I need to reconcile these two documents.

The idea of merging this before it's truly complete comes about because after tomorrow (May 14) I will have less time to spend on it and it will be better if I can break any remaining work up into smaller units by starting new PRs. There have been inconsistencies in the documentation for some time and I don't think the merge would make things worse. @rabbah let's consult on timing.

[joshuaauerbachwatson]

• the old configuration section still exists and is still referred to. Thus, many configuration options are documented twice.

I removed the reference-like material from configuration.md and adjusted any specific links from elsewhere to point to the spec document instead. But, configuration.md remains for its overview and example sections and is still pointed to from other documents where it seems like a high-level overview is called for.

I still have the outtakes from configuration.md and am starting to comb through them for useful explanations that ought to be folded into the spec. Once that pass is over I will consider this work complete for the moment.