docs: update build environment requirements

[DurvalMenezes]

Summary

Update the compilation section of the README with current information about supported build environments.

Changes

• Document Debian 13 Trixie and Ubuntu 26.04 as known-working environments.
• Clarify that other distributions should work with reasonably recent AVR toolchain packages, and list known-good versions of these packages so anyone using other distros can try and install those.
• Add a link to the Docker build environment, ditto to its currently known issues.

This is documentation-only; no firmware behavior is changed.