Introduce context variables

[Mpdreamz]

Today we support

• global variables through docset.yml
• yamlfront matter on pages.

As we move forward with migrating our asciidoc documentation over there are also various variables that the docs-builder should expose to all docsets e.g:

• {branch} current branch information.

TODO list all that we find here.

I would want to expose these types of variables under a common context.* prefix e,g: {context.branch}

This to insure no conflicts will occur with user defined variables as well as making it clearer these are variables defined by the system not declaratively.