Migrate Netsurf 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 and include them in the documentation. Raising this before we forget.

There are some issues with the projects currently which stops them building, which will need fixing before we migrate. So this ticket will need to be updated once these are better understood.

Task description

• Migrate the Netsurf example

Acceptance Criteria

• Link to documentation page showing X86 image example
• Ensure that the user is instructed to install BuildStream-external via pip as they will require the flatpak_image plugin