Document barectf 3

Document barectf 3

This patch adds the complete documentation of barectf 3 (upcoming major
release of the project).

The new `docs` directory is an Antora [1] documentation component
version. You can include it in an Antora site.

All the pages are written in AsciiDoc (Asciidoctor version).

Some contents were copied (and adapted) from the barectf wiki [2], but
most of the pages feature brand new text. The logo and diagrams are also
refreshed.

I'm removing the `barectf` CLI tool's manual page (`doc/man`) as it's
now documented in `docs/modules/cli/pages/usage.adoc`. Having the CLI
documented within the whole project's documentation makes it possible to
have critical links to and from other parts of it. This is to the
advantage of the reader.

[1]: https://antora.org/
[2]: https://github.com/efficios/barectf/wiki