doc: copy dev guide to hledger repo, clarify manual/devguide status

The plan is to keep a small number of key docs in both the main hledger
repo and the hledger.org wiki, and merge the updates from both sides
periodically (at release time and on the first of the month, eg.)
Because sometimes you're working on the web, sometimes you're working in
the code, and I can't give up either one.