zerocloud: minor tweaks/revisions to dev env docs
Merge pull request #43 from larsbutler/dev-env-tweaks

zerocloud: minor tweaks/revisions to dev env docs
Complete rewrite of ZeroCloud overview
index: change "getting start" to "contributing"
index: move "contributing" section to the bottom

This leaves topics which are more relevant to most developers at the top
of the page.
index: update "welcome" heading
clitools: update page title

Replaced "CLI" with "Command Line".
clitools: replace Zwift with ZeroCloud

Zwift has long been deprecated.
clitools: reorganize/rewrite

- zpm is at the top, instead zvsh/zvapp (zerovm-cli)
- Reworded the subproject links to zpm and zerovm-cli
zerocloud/overview: update overview heading
Add ZeroVM core docs
index: Reorganize index completely, plus new intro
zerocloud/overview: minor revision/rephrasing