docs: Convert GVariant Specification to reStructuredText

This is a verbatim conversion of the GVariant Specification from https://people.gnome.org/~desrt/gvariant-serialisation.pdf / https://raw.githubusercontent.com/ostreedev/gvariant-rs/master/docs/gvariant-serialisation.pdf to reStructuredText.

This is for several reasons:

1. The canonical copy has gone offline due to people.gnome.org being shut down.
2. GLib is the reference implementation of GVariant, so should probably host the specification (unless someone wants to host it on freedesktop-specs).
3. Moving it out of a PDF and into reStructuredText should allow for amendments. The specification has a few problems, typos and oversights in its original form, and it would be good to canonically fix them without having to write a separate addendum document.

This conversion is verbatim, and does not change the content of the document at all, even to fix typos and broken links (which there are a small number of in the PDF).

This describes what I’m labelling as version 1.0 of the GVariant serialisation format. Updates to the specification can bump this version number, in subsequent commits.

Signed-off-by: Philip Withnall pwithnall@endlessos.org