Add how-to page for callback groups

[tanelikor]

Based on the discussion in ROS Discourse,
converted the post into documentation from and added it as a how-to page.

Also added couple short rclpy-side notes on the Callback Groups section of About Executors page, but only on the callback group section for now.

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/how-to-use-callback-groups-in-ros2/25255/8

[ralph-lange]

Very nice write-up. Thank you very much! I propose minor changes only.

Note: I first considered extending the “Avoiding deadlocks” section by a discussion of the tradeoff between conservative configurations (one or few callback groups with risk of deadlocks and poor utilization of computing resources) vs. optimistic configurations (multiple callback groups or reentrant callback groups with risk of data races), but there is much less to write about the data race risk and it’s already touched in the previous section. Therefore, let’s keep the last section focused on deadlocks and synchronous calls as it is.

[ralph-lange]

LGTM

[tanelikor]

Thanks for the review and suggestions. I committed the suggested changes.

[fujitatomoya]

I believe this explains really well. lgtm with minor nitpicks.

[clalancette]

In short, this is fantastic. Thanks so much for doing this.

I pushed one small cleanup commit, since we don't need a redirect for a new page. Other than that, this looks great so I'll approve and once CI comes back clean, I'll go ahead and merge it in.

[tonynajjar]

Thanks a lot for this tutorial. In addition to this I attempted to create a "decision table" internally for my company and I wonder if you would find this useful to somehow insert in the tutorial:

Additionally, I really like the last section "Examples" and think we could add more scenarios:

If you agree that these additions are useful, I could create a PR. Let me know.

P.S: this is the result of my personal experimentation so there might be mistakes, happy to get corrections!

[clalancette]

Please feel free to open a new PR; we can iterate there.