Add example with build directory outside of source directory

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/716)
In GitLab by [[Gitlab user @phildawson]](https://gitlab.com/phildawson) on Oct 16, 2018, 13:37

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

A left over item following the merge of !776

This item describes Technical debt.

See for https://gitlab.com/BuildStream/buildstream/merge_requests/776#note_107972976 the original comment.
## Task description

[//]: # (Short summary of the action to be executed)
Add an example about "How to place your build directory outside of the source directory" example, which might contain multiple elements to show how to do it with cmake or autotools.

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

There is an example in doc/examples showing how to place the build directory of an element outside of it's source directory.

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Start in doc/examples and read the linked GitLab merge request discussion for the context from !776. Add an example showing a build directory outside its source directory, using multiple elements and covering the CMake or autotools cases described. Done means the example is present in doc/examples and demonstrates the requested layout.

Written by the indexing model from the issue text.