Run MD tests for Markdown-only changes (#19479)

## Summary

Exclusions in Git pathspecs [are not
order-sensitive](https://css-tricks.com/git-pathspecs-and-how-to-use-them/#aa-exclude):

> After all other pathspecs have been resolved, all pathspecs with an
exclude signature are resolved and then removed from the returned paths.

This means that we can't write chains like we had here before to exclude
Markdown file changes *unless* they are in
`crates/ty_python_semantic/resources/mdtest`. This doesn't work. The
exclude pattern will just overwrite the second pattern and all Markdown
changes will be excluded:

```bash
':!**/*.md' \
':crates/ty_python_semantic/resources/mdtest/**/*.md' \
```

The configuration we had here before meant that tests wouldn't run on
MD-test only PRs, see e.g. #19476.

So here, I'm proposing to remove the broad `:!**/*.md` pattern. We can
always add more fine-grained exclusion patterns, if that's needed. The
`docs` folder is already excluded.

## Test Plan

Tested with local `git diff` invocations.

Author: sharkdp

.github/workflows/ci.yaml

@@ -143,12 +143,12 @@ jobs:
         env:
           MERGE_BASE: ${{ steps.merge_base.outputs.sha }}
         run: |
-          if git diff --quiet "${MERGE_BASE}...HEAD" -- ':**' \
-            ':!**/*.md' \
-            ':crates/ty_python_semantic/resources/mdtest/**/*.md' \
+          # NOTE: Do not exclude all Markdown files here, but rather use
+          # specific exclude patterns like 'docs/**'), because tests for
+          # 'ty' are written in Markdown.
+          if git diff --quiet "${MERGE_BASE}...HEAD" -- \
             ':!docs/**' \
             ':!assets/**' \
-            ':.github/workflows/ci.yaml' \
           ; then
               echo "changed=false" >> "$GITHUB_OUTPUT"
           else