Update README.adoc

Update README.adoc

Notable changes:

‣ Fix broken LTTng CI links.

‣ Reword a few sentences to improve clarity.

‣ Use mostly "Babeltrace 2".

‣ Remove the sentence saying Babeltrace 2.0 is a "recent" release.

‣ Add an image (a conversion graph diagram) to maintain attention.

‣ Add a ToC, placing it at a specific location after the preamble when
  GitHub renders.

‣ Add thanks to the authors of embedded projects.

‣ Mention `--enable-built-in-plugins` and
  `--enable-built-in-python-plugin-support`.

‣ Transform the note in "Community" to regular text.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I487d781f435322652fde44cf1fed66f8a4d07c72
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11748