Fuzzy matching (and suggestions?)

Add a mode to fuzzy match headlines with a list of known headlines (see [this post](https://deterministic.space/machine-readable-inline-markdown-code-cocumentation.html) for a proposal) and suggest fixes for close matches.

Something like this `Suggestion { found: "Example", suggested: "Examples", … }`. (Fields tracking span data etc omitted. Should give enough details to be usable by clippy.)

Start by reading the linked proposal and reviewing the rust-docstrings project to determine how known headlines and clippy diagnostics should be represented. Define the fuzzy-matching mode and a usable Suggestion result, including the omitted span-tracking fields, then verify that close matches produce actionable fixes for clippy.

索引モデルが issue の本文から書いたものです。