Update configuration guide

[victorlin]

• [ ] Document predefined subsampling schemes all, region, region_global, region_grouped_by_country, country, division, location from https://github.com/nextstrain/ncov/pull/894#discussion_r832704630
• [ ] prohibit certain suffixes in builds: entry names from https://github.com/nextstrain/ncov/pull/896#discussion_r836882802
  1. Update config.schema.yaml
  2. Update Snakefile
  3. Update configuration guide
  4. Add a note to sync the above three and nextstrain.org/src/utils/index.js