Split Sidekiq development docs into separate files

`sidekiq_style_guide.md` was becoming very large, and was no longer
simply a style guide. This splits it into separate files by topic:

1. An overview page (`index.md`).
2. Compatibility across updates (`compatibility_across_updates.md`).
3. Job idempotency (`idempotent_jobs.md`).
4. Limited capacity worker (`limited_capacity_worker.md`).
5. Logging (`logging.md`).
6. Worker attributes (`worker_attributes.md`).

It does not change any content, other than to fix link targets.