Merge docs-builder and docs-assembler

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.