LiquiDoc CMF V3 Bootstrap & Boilerplate

This PR releases version 3 of the LiquiDoc Content Management Framework.

* The Readme has been wholly upgraded, mostly for clarity but also in preparation for serving as source for external repos, notably the [WIP "LiquiDoc CMF Guides" project](https://github.com/DocOps/liquidoc-cmf).

* The comment-suppressed `jekyll` block of the boilerplate `Gemfile` has been uncommented and thus will install with `bundle install`/`bundle update` commands.

* Adds templates for initiating topics. The new commands generate a file with default/templated content as well as print to console a chunk of YAML for including in a `manifest.yml` file.

* Templatizes files that were in `_configs/` directory in V2, now built during init only.

* Lots of clarifying work in `data/meta.yml`.

* Clarifies bootstrapping instructions, which have been tested in multiple environments.

* Adds an HTML edition of the default document and tidied up the build.

* Huge thanks and ++ to @diggerlilly for thorough review under pressure. We're getting close to this thing being user friendly in the ways it deserves!