New Feat: add rss

[ZeddYu]

Summary

• Add RSS feed generation script (convert_to_rss.py) that produces RSS 2.0 XML feeds from conference YAML data,
  mirroring the existing iCal generation pipeline
• Add RSS feed section to the subscription modal, alongside existing iCal links, with copy buttons and bilingual
  support (en/zh)
• Add RSS autodiscovery tag to index.html for browser/extension detection
• Update CI workflows (deploy.yml, test-build.yml) to generate and verify RSS feeds
• Related issue: 添加rss订阅功能 #684

Details

• RSS script reuses load_mapping(), get_timezone(), reverse_index() from convert_to_ical.py, uses only Python stdlib
  (no new dependencies)
• Feed files follow the same naming convention as iCal: deadlines_{lang}.xml, deadlines_{lang}{sub}.xml,
  deadlines{lang}{rank}{sub}.xml, etc.
• Each RSS item includes conference metadata (date, location, rank, DBLP link) and uses deterministic GUIDs

[mergify]

Welcome to ccf-deadlines!
We're delighted to have you onboard (p≧w≦q)

[mergify]

Great job!
Thank you @ZeddYu for your contribution!