Changes from 2.11:
* "What's new?" section.
* Installation section.
* Document the `lttng clear` feature.
* Document the process attribute tracking feature (update the existing
PID tracking section).
* Add the "inclusion set", "process attribute", "tracked process
attribute", and "untracked process attribute" terms to the glossary.
* Convert all Babeltrace 1 references/examples to Babeltrace 2 ones.
* Fix English style issues.
* Convert a few simple lists to description lists where appropriate.
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>