docs: describe potential length extension attack when using MiMC in-circuit

[ivokub]

Description

Certain use cases of MiMC may lead to length extension attacks. Document these cases to help the users avoid them.

This will follow up with a PR on gnark-crypto to give the same considerations.

Fixes #1141

Type of change

• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules