Add what is the default concurrency value to the documentation

[kustowski1]

Bug Report

Describe the bug
I was asked to put a request to add the default concurrency value to the documentation as a bug report.

Additional context
The documentation should mention that the default concurrency is the number of CPUs on the system: https://docs.celeryq.dev/en/latest/reference/cli.html#cmdoption-celery-worker-c

[koning]

The default concurrency is the total number of PUs available on the node, e.g. 72 for a 36 core CPU with 2 hardware threads.

You can see the concurrency value of your workers by running:
celery -A merlin inspect stats | grep concurrency

[bgunnar5]

This was added in the v1.12.0 release when the documentation was overhauled to mkdocs