ci: exclude docs/run_maxtext.md from mdformat

mdformat-myst escapes the :::{grid} colon-fence directives in this file,
which breaks sphinx-design grid rendering in the docs build. The grid
predates this PR; CodeQuality only flags it now because the PR touches
the file (pre-commit runs on changed files only). Mirrors the existing
exclusions for docs/guides.md and docs/guides/checkpointing_solutions.md.

Author: inardini

.pre-commit-config.yaml

@@ -60,7 +60,7 @@ repos:
         args: ['--number']
         additional_dependencies: [mdformat-myst, mdformat-ruff]
         files: (docs/.)
-        exclude: docs/guides/checkpointing_solutions.md|docs/guides.md
+        exclude: docs/guides/checkpointing_solutions.md|docs/guides.md|docs/run_maxtext.md
 
   - repo: https://github.com/adrienverge/yamllint
     rev: v1.35.0