SUP-2217: highlight block and input steps cannot be used within concurrency gates

[tomowatt]

We had user mention this information is not emphasied enough as it is a short line added to the end of the paragraph and easily missed resulting in errors when using concurrency gates, so moving it into its own block to highlight it.

[buildkite-docs-bot]

Preview URL: https://2799--bk-docs-preview.netlify.app

[pzeballos]

@gilesgas this is the same as #2620

[gilesgas]

Thanks @tomowatt and @pzeballos , I will rework the wording and implementation of this new callout now that we have the new troubleshooting callout (with the relevant info) above it in the previous section. I'd like to re-work the wording in this first callout a tad too as I feel we overload the word 'block' too much.

[gilesgas]

OK @tomowatt and @pzeballos ,

To get this PR merged in a reasonable timeframe (and while my mind is fresh on this topic), I've pushed some wording updates directly to this PR.

Could please review these wording updates and let me know if they're accurate or not?
If they're incorrect, please let me know and how to fix the wording to make it correct.

If you don't have time to do this, then please let me know:

• Are concurrency groups and concurrency gates the same thing?
• If not, are concurrency gates a specific implementation of concurrency groups?

[gilesgas]

I need answers to the queries in my previous comment before approving this PR.

[pzeballos]

• Are concurrency groups and concurrency gates the same thing?

Kind of

• If not, are concurrency gates a specific implementation of concurrency groups?

Correct. Concurrency gates is a concept that you use, for example, to lock deployment either to an environment, or to end-to-end tests and the way to do it is with concurrency groups.
It's particularly useful for managing resources, ensuring that certain critical paths in your pipeline do not overwhelm shared resources, and preventing race condition