Update built-in timer documentation

[jessica-mitchell]

This PR adds a diagram describing a simplified simulation run focusing on timers.
The diagram is generated via a script that converts YAML into a dot diagram. Thanks to @JoseJVS for providing the original version.
In addition, some links are added to try to connect various parts of the documentation together with respect to perforance, benchmarking, and timers.

https://nest-simulator--3490.org.readthedocs.build/en/3490/nest_behavior/built-in_timers.html#built-in-timers

[mlober]

Thank you for this, it already looks really good. I had a look at the code again and have a few small improvements:

• diagram: change "Repeat once if buffer too small" to "Repeat once if not all spikes transmitted".
• diagram: block "if buffer can shrink" must be outside of "Repeat once if not all spikes transmitted".
• diagram: include another block after time_communicate_spike_data-block that is the opposite of "if buffer can shrink": "if buffer too small -> grow MPI buffer size".
• OMP parallel and OMP master with same capitalization
• question to @JanVogelsang : shouldn't the OMP parallel end before OMP master?

[JanVogelsang]

How about making the timer block backgrounds nest-orange instead of white?

shouldn't the OMP parallel end before OMP master?

No that's correct, OMP master sections only make sense within parallel sections, outside of them master is the only thread running anyway

[JanVogelsang]

I'm not sure if "Repeat once if not all spikes transmitted" is actually better than "Repeat once if buffer too small". That makes it sound as if in the first round, the communicated spikes are already used and only the remaining spikes are communicated in the second round.

[mlober]

Keeping it "Repeat once if buffer too small" is fine as well. But I would then write it as "Repeat once if spike buffer too small", since the reader might wonder what this "buffer" is.
One more thing: Jessica and I agreed to remove the blocks about resizing the buffers, since it is not essential to understand the timers and reduces complexity.