Docs: Refresh Forgejo project documentation

[whereareiam]

Summary

• Rebrand the README and issue template for the Forgejo-focused fork.
• Document release, development-build, and local asset installation paths.
• Remove obsolete Chinese documentation, standalone changelogs, and translation generation.
• Add the updated dashboard screenshot and align release packaging with the removed translation archive.

Verification

• bun check
• git diff --cached --check