Migrate blab/styleguide to the Snakemake style guide

[victorlin]

Description of proposed changes

This old styleguide contains recommendations that are relevant to the pathogen workflows presently maintained.

Related issue(s)

Prompted by https://github.com/nextstrain/docs.nextstrain.org/pull/162#pullrequestreview-1546026324.

Checklist

• [x] Checks pass
• [x] Preview page looks good
• [x] Post-merge: update the nextstrain-builds.md document to link to the new page
• [x] Post-merge: update the README to say this is unused/never used, and archive the repo (so old links still work)

[victorlin]

@tsibley thanks for doing that, changes look good! Merging.