Exclude *.md files from formatting and linting

[KatieB5]

Python embedded in Markdown is now automatically picked up by Ruff. 15c48a3 added a just check workflow in CI. We want this to run for PRs merging into main, but don't want researchers who contribute directly to the documentation to encounter failing checks due to formatting or linting of Python snippets in Markdown (discussion in Slack thread).

This PR excludes *.md files from Ruff formatting and linting.

Local testing

Introduce a formatting error in a .py file, and a python snippet in a .md file:

(.venv) $ just check
/home/katie/bin/just _compile requirements.prod.in requirements.prod.txt 
/home/katie/bin/just _compile requirements.dev.in requirements.dev.txt 
$BIN/ruff format --check .
unformatted: File would be reformatted
   --> docs/case-control-studies.md:183:6
    |
182 |         "index_date": datetime.date,
    -     }
183 +     },
184 | )
    |

invalid-syntax: Unexpected indentation
  --> hooks/ehrql_branch.py:17:1
   |
15 |     """
16 | ehrql_branch = config["extra"]["ehrql_branch"]
17 |     if ehrql_branch == "main":
   | ^^^^
18 |         return config
19 |     ehrql_nav_index, ehrql_nav_section = next(
   |

1 file would be reformatted, 109 files already formatted
error: recipe `format` failed on line 112 with exit code 2

Update pyproject.toml to add the exclude-extend setting and exclude markdown files.

(.venv) $ just check
/home/katie/bin/just _compile requirements.prod.in requirements.prod.txt 
/home/katie/bin/just _compile requirements.dev.in requirements.dev.txt 
$BIN/ruff format --check .
invalid-syntax: Unexpected indentation
  --> hooks/ehrql_branch.py:17:1
   |
15 |     """
16 | ehrql_branch = config["extra"]["ehrql_branch"]
17 |     if ehrql_branch == "main":
   | ^^^^
18 |         return config
19 |     ehrql_nav_index, ehrql_nav_section = next(
   |

6 files already formatted
error: recipe `format` failed on line 112 with exit code 2

Only the .py formatting error is picked up.

NOTE: Once this PR is merged, branch protection rules will need to be updated in the docs repo Settings, to enforce the new just check CI workflow to run for PRs to merge into main.

TO DO:

• Update branch protection rules (Slack thread confirmation)

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