Migrate Netsurf example from examples repo to main repo

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/439)
In GitLab by [[Gitlab user @LaurenceUrhegyi]](https://gitlab.com/LaurenceUrhegyi) on Jun 22, 2018, 11:57

## 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](https://gitlab.com/BuildStream/buildstream-examples/tree/master/netsurf-flatpak)

## 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

Review the linked netsurf-flatpak example and the original GitLab issue first; the issue says the current projects do not build and need further investigation. Done means the example is migrated, a documentation page links to an X86 image example, and users are told to install BuildStream-external via pip.

Written by the indexing model from the issue text.