Merge docs-builder and docs-assembler

[Mpdreamz]

The two CLI tools existed primarily to create a hard boundary to enforce we build for distributed isolated builds first and assemble over these distributed pieces.

Now that we are slowly refactoring Elastic.Markdown into more dedicated assemblies we can ensure the assembler bits live in a separate assembly without having to go through a different CLI tool to call into it.

docs-builder assemble source
docs-builder assemble build
docs-builder assemble 

With the latter being the same as calling source and build in succession.