docs: add GLINR Discord community links

[thegdsks]

Summary

The official GLINR Discord server just launched. This adds community links so users can find it from the repo.

• README.md: added a Discord badge to the existing badge row (matches the plain shields.io style of the other badges, no style param).
• README.md: added a "Community" section before "Contributing", with a short blurb and the GLINR Discord widget banner. Mentions this project's forum channel is #sslx.
• .github/ISSUE_TEMPLATE/config.yml: added a new contact_links entry pointing to the Discord invite, alongside the existing Discussions link.

Does not do

• Does not touch Cargo.toml, Cargo.lock, or any other packaging metadata.
• Does not modify CI workflows.
• Does not touch CONTRIBUTING.md (no "questions" / "getting help" section exists there to anchor a pointer, so it was left alone per the task instructions).
• Does not touch any sub-crate README (this repo is a single Rust CLI crate with no separate published-package README).

Test plan

• Visual check of the rendered badge row and Community section on GitHub

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