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.

**Additional context**

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

**Does this issue could impact on users private data?**

No

**Funded by**

No