Migrate X86 image example from examples repo to main repo

Background

Instead of having a separate repo for examples of BuildStream, we should migrate these to the main repo, or the bst-external repo, and include the relevant documentation.

Task description

• Decide to migrate to main repo or bst-external
• Solve bst-external#3 (moved)
• Update and amend the docs we currently have for authoring an image - !262 (closed)
  • Give the example a "name"
  • Create doc/source/examples_${name}.rst
  • Ensure we use the "literalinclude" rst directive to show the example .bst files
  • Ensure we link to the correct pages when talking about features
  • Ensure that we instruct the user to install bst-external (via pip) onto their system as this example makes use of the x86image and docker plugins
• Migrate the X86 image example
  • Create a sample project that lives in the directory doc/examples/${name}/
  • Create an integration test which runs in CI for the example, this is called tests/examples/${name}.py

Acceptance Criteria

• Integrate the x86image example into the documentation