Add initial Python bindings documentation

This initial documentation contains a home page, an installation page,
and a few examples to understand how the `bt2` package works.

Still missing: how exactly the bindings wrap libbabeltrace2 (wraping
rules, exceptions, etc.).

Changes:

`README.adoc`:
    Specify that you need Sphinx to build the Python bindings
    documentation.

`configure.ac` and `m4/check_sphinx.m4`:
    Add `--enable-python-bindings-doc` which requires
    `--enable-python-bindings`.

    This is because the Sphinx configuration file actually imports the
    `bt2` package to get the version (and, eventually, for Sphinx's
    autodoc to find docstrings within the `bt2` modules).

`doc/bindings/python/source`:
    The actual documentation's contents and configuration.

`doc/bindings/python/ext/bt2sphinxurl.py`:
    A Sphinx extension to add Babeltrace 2 manual page and other links
    of which the URL includes the project's version.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4811336d567ff379cbe9e789099af8d6c7661a62
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3278
Tested-by: jenkins <jenkins@lttng.org>