docs: Partially document the liblttng-ctl C API

This patch:

1. Performs the required changes to make the build system able to build
   an HTML API documentation using Doxygen.

   The way it's done is a replica of what does the Babeltrace 2 project,
   which you may be familiar with.

   `doc/api` is for all API documentation projects while
   `doc/api/liblttng-ctl` is the specific liblttng-ctl API documentation
   project.

   To build and view the HTML API documentation:

   a) Configure the project with the `--enable-api-doc` option.

   b) Build and install the project.

   c) Open
      `$prefix/share/doc/lttng-tools/api/liblttng-ctl/html/index.html`,
      where `$prefix` is the installation prefix (for example,
      `/usr/local`).

2. Fully documents some modules while not documenting others at all.

   Because some liblttng-ctl headers contain functions/types which
   conceptually belong to more than one module (unlike in Babeltrace 2),
   I decided to put all the Doxygen group (module) definitions and any
   "extra" module documentation in `dox/groups.dox`. The latter is a
   huge file of which most of the contents was copied from the
   LTTng-tools manual pages (and from the online LTTng Documentation)
   and adapted to the C API context.

   Images are direct copies from the LTTng Documentation.

   The complete module tree and its state, as of this patch, is as
   follows, where ✅ means it's fully documented and ❌ means it's not
   documented at all:

       ✅ Home page

       ✅ General API (error codes, session daemon connection,
          common definitions)

          Includes parts of `lttng.h`, `lttng-error.h`, and
          `constant.h`.

       ✅ Recording session API

          Includes parts of `lttng.h`, `channel.h`, `handle.h`,
          `domain.h`, and `session.h`.

          ✅ Recording session descriptor API

             Includes all `session-descriptor.h`.

          ✅ Recording session destruction handle API

             Includes all `destruction-handle.h`.

          ✅ Domain and channel API

             Includes parts of `channel.h`, `domain.h`, and `event.h`.

             ✅ Recording event rule API

                Includes parts of `event.h`.

          ❌ Process attribute inclusion set API

             Would include parts of `tracker.h`.

          ✅ Recording session clearing API

             Includes all `clear.h` and `clear-handle.h`.

          ❌ Recording session snapshot API

             Would include all `snapshot.h`.

          ❌ Recording session rotation API

             Would include all `rotation.h` and `location.h`.

          ❌ Recording session saving and loading API

             Would include all `save.h` and `load.h`.

       ✅ Instrumentation point listing API

          Includes parts of `event.h`.

       ❌ Trigger API

          Would include all `trigger/trigger.h`.

          ❌ Trigger condition API

             Would include all `condition/buffer-usage.h`,
             `condition/condition.h`, `condition/evaluation.h`,
             `condition/session-consumed-size.h`, and
             `condition/session-rotation.h`.

             ❌ "Event rule matches" trigger condition API

                Would include all `condition/event-rule-matches.h`.

                ❌ Event rule API

                   Would include all headers in `event-rule` as well
                   as all `kernel-probe.h` and `userspace-probe.h`.

                   ❌ Log level rule API

                      Would include all `log-level-rule.h`.

                ❌ Event expression API

                   Would include all `event-expr.h`.

                ❌ Event field value API

                   Would include all `event-field-value.h`.

          ❌ Trigger action API

             Would include all `action/action.h`,
             `action/firing-policy.h`, `action/list.h`, `action/path.h`,
             `action/rate-policy.h`, `action/rotate-session.h`,
             `action/snapshot-session.h`, `action/start-session.h`, and
             `action/stop-session.h`.

             ❌ Notify trigger action API

                Would include all `action/notify.h`,
                `notification/channel.h`, and
                `notification/notification.h`, as well as parts of
                `endpoint.h`.

       ❌ Error query API

          Would include all `error-query.h`.

   I'm voluntarily not documenting the health API (`health.h`), as I
   believe it's not super important for most users. We could document it
   on demand.

   In `groups.dox`, the groups of the undocumented modules are already
   defined, so that the complete tree above is visible in the rendered
   "API reference" section. The undocumented modules simply show the
   text "To be done". Because there are references to undocumented
   modules in `groups.dox` and in the documented headers, this means
   that the links at least resolve.

   Note that there are non-comment changes in `include/lttng`: I needed
   to name some anonymous, nested types so that I could reference their
   members, as you can only link to the member of a named type with
   Doxygen. For example, the type of the `u` union member of
   `struct lttng_event_context` is now `union lttng_event_context_u`;
   then you can reference its `probe` member as such:
   `lttng_event_context::lttng_event_context_u::probe` (_not_
   `lttng_event_context::u::probe`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I2783419159f4892a992fe5bc760b6e2cd6d13a60