chore: remove Discord links and mentions from site and docs

[omaiesh]

Summary

• Remove Discord community link, nav button, footer entries, and the Discord-themed feedback button from the marketing site and docs layouts.
• Drop Discord-specific CSS (nav a.discord-link, .rm-feedback-btn--discord and their light-theme/mobile variants) from docs/web/assets/styles.css.
• Remove Discord bullet from README.md, USAGE_GUIDE.md, TROUBLESHOOTING.md, sub-package READMEs, and Discord = ... URLs from each pyproject.toml.
• Remove "collaboration": "Discord" and "collaboration_project_invitation_link" from gain.json.

Test plan

• grep -ril discord . returns nothing outside .git.
• python3 -c "import json; json.load(open('gain.json'))" succeeds.
• Build the Jekyll site locally and confirm nav, footer, contribute and roadmap pages render without missing-style artifacts.

🤖 Generated with Claude Code

[github-actions]

Rosetta Triage Review

Summary: This PR removes all Discord links, nav buttons, footer entries, Discord-themed CSS, and Discord-specific gain.json/pyproject.toml fields from the marketing site, documentation, and configuration — a clean, focused deprecation of the Discord community channel.

Findings:

• Scope is well-contained: All 19 files changed are directly related to Discord removal. No unrelated concerns are mixed in.
• CSS cleanup is thorough: Dark-theme, light-theme, mobile-responsive, and NAV underline override variants of .discord-link and .rm-feedback-btn--discord are all removed.
• HTML removal is consistent: Nav entries removed from nav.html, default.html, and docs.html; inline reference in contribute.md also updated.
• Config cleanup: gain.json removes both collaboration and collaboration_project_invitation_link fields; all pyproject.toml files drop the Discord URL.
• No logic changes: No new code paths introduced; no automated test changes are needed.
• Test plan is present: PR body includes three verification steps (grep, JSON parse, Jekyll build).
• Minor note: gain.json field removals (collaboration, collaboration_project_invitation_link) are a config schema change. No functional breakage is expected since these fields are community metadata only, but reviewers may want to confirm no internal tooling reads them.

Suggestions:

• Consider running grep -ril discord . locally before merge to ensure no stragglers remain (as noted in the PR test plan).
• The collaboration_project key in gain.json still references "Rosetta Community" — if there is no replacement community platform planned, this field may also be a candidate for removal or update.

Automated triage by Rosetta agent