Update configuration guide

[victorlin]

• Document predefined subsampling schemes all, region, region_global, region_grouped_by_country, country, division, location
  from docs: convert to reST, re-organize and update contents across all pages #894 (comment)
• prohibit certain suffixes in builds: entry names
  from doc: convert config guide to reST, document name restrictions for builds: #896 (comment)
  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