Update KNOWN_USERS.md

[DeVikingMark]

Description

This pull request updates the KNOWN_USERS.md file to include detailed information about the Linea project, which uses gnark in production. The changes highlight Linea’s use cases, including precompile proving, recursive verification, proof aggregation, and proof compression, along with the technologies used (gnark and gnark-crypto).

Fixes # (N/A - this PR does not address a specific issue).

Type of Change

• Documentation update

How Has This Been Tested?

This is a documentation update and does not require code testing. The changes have been manually reviewed for accuracy and consistency.

How Has This Been Benchmarked?

N/A - No benchmarks required for documentation updates.

Checklist:

• I have performed a self-review of my changes.
• I have commented my changes where necessary.
• I have made corresponding changes to the documentation.
• This change does not modify any generated files.
• New and existing unit tests are not applicable.
• Any dependent changes have been merged and published in downstream modules.