Update Ruff mdtest structure (#25246)

Summary
--

While working on #25233, I realized that my suggestion to group multiple
rules within the same
Markdown file using top-level headings ran afoul of a markdownlint
[rule] against having multiple
top-level headings in the same file. I was a bit worried about the size
of the resulting files if we
followed this suggestion anyway, so I think the best course of action is
to recommend directories
for each linter with a separate file per rule.

This PR updates our existing mdtests to this new structure and also
updates the recommendation in the
contributing docs.

Test Plan
--

Existing mdtests in new locations

[rule]:
https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md025---multiple-top-level-headings-in-the-same-document

Author: ntBre

CONTRIBUTING.md

@@ -234,9 +234,9 @@ preserving the original rule name.
 
 To test rules, Ruff uses the mdtest framework, initially developed for ty. Mdtests are written as
 Markdown files with Python code and TOML configuration blocks alongside prose
-descriptions. Generally, there will be one file per linter (e.g. `flake8-bandit.md` for the `S`
-rules) with top-level headings for each rule (e.g. `S704` for `unsafe-markup-use`). Within these
-top-level sections, you can define additional sections to group related tests and their settings
+descriptions. Generally, there will be one directory per linter (e.g. `flake8-bandit` for the `S`
+rules) with nested files for each rule (e.g. `unsafe-markup-use.md` for `S704`). Within these
+files, you can define additional Markdown sections to group related tests and their settings
 together.
 
 You can see [the ty_test
@@ -252,7 +252,7 @@ For example, a minimal mdtest for `module-import-not-at-top-of-file` (`E402`) wo
 like this:
 
 ````markdown
-<!-- crates/ruff_linter/resources/mdtest/pycodestyle.md -->
+<!-- crates/ruff_linter/resources/mdtest/pycodestyle/module-import-not-at-top-of-file.md -->
 
 # `module-import-not-at-top-of-file` (`E402`)