[mgiovani]

Summary

The README grew to 721 lines, with every rule, the corpus tables and the full config reference inline. This PR turns it into a 117-line pitch (what stopslop is, install, one real run, why, groups, minimal config, CI, non-goals) and moves the long-form content, verbatim, into docs/:

Page	Holds
docs/README.md	index
docs/why.md	intro, why existing linters miss this, full non-goals
docs/usage.md	install variants, usage, git-aware selection, baseline, exit codes
docs/rules.md	rule groups, tier counts, the per-rule table
docs/prose.md	prose linting
docs/configuration.md	config file, path exemptions, user-defined rules, --check-imports
docs/suppression.md	suppression directives
docs/benchmarks.md	corpus measurements, human splits, candidate tells

These files map one to one onto pages of the docs site in #101.

Other changes

• --help footer points at docs/README.md.
• SARIF helpUri points at docs/rules.md. The old README.md#slopNNN fragment never resolved, because the table has no per-code anchors; docs: rule reference site with one page per rule #101 adds real per-rule pages.
• bench/score_corpus.py (--readme becomes --rules-doc) and bench/corpus/results.py read the rule table from docs/rules.md. It parses 51 rows, matching --list-rules.
• AGENTS.md: "update the README rule table" becomes docs/rules.md.
• README links are absolute, so they also work on crates.io.

Verification

• A normalized-line diff of the old README against the new README plus docs/*.md drops nothing. The 11 changed lines are reflowed text or links that now cross files.
• Every relative link and anchor in README.md and docs/*.md resolves.
• just check passes. stopslop README.md docs/ reports zero findings.

Merge-order note: The feat/design-style-layer branch (#86, #87) adds rule rows to the README table. Whichever of the two lands second moves those rows into docs/rules.md.