Fix markdownlint configuration to exclude external directories

- Add ignores to .markdownlint-cli2.yaml for devito_repo/, venv/, etc.
- Disable MD018 (shebang false positives) and MD036 (exercise labels)

Author: ggorman

.markdownlint-cli2.yaml

@@ -3,3 +3,13 @@
 globs:
   - "**/*.md"
   - "**/*.qmd"
+
+# Exclude external and generated directories
+ignores:
+  - "devito_repo/**"
+  - "venv/**"
+  - ".venv/**"
+  - "node_modules/**"
+  - "_book/**"
+  - "doc/pub/**"
+  - "devito-plan.md"

.markdownlint.yaml

@@ -53,6 +53,12 @@ MD022: false
 # MD012 - Multiple consecutive blank lines (false positives in Python code blocks with PEP8 style)
 MD012: false
 
+# MD018 - No space after hash (false positive from #!/bin/bash shebangs in code blocks)
+MD018: false
+
+# MD036 - Emphasis as heading (false positive from a) b) c) exercise labels)
+MD036: false
+
 # MD004 - Unordered list style (allow both dash and plus - converted from DocOnce)
 MD004: false