Support placeholder for username and nickname
**Is your feature request related to a problem? Please describe.**
I think it is difficult to understand the difference between a user name and a nickname.
**Describe the solution you'd like**
It would be nice to use the form's placeholder to display an example input.
**Describe alternatives you've considered**
You could also write it in the description of each item, but I think it would be a bit dense.
In the instance I'm working on, We're trying to implement the above by rewriting the template. I would like to see it adopted in the mainstream as well.
See also my pull request: https://github.com/decidim/decidim/pull/7380
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