Clear glossary language directories before regeneration to remove stale entries

[Copilot]

When glossary entries are deleted from the source spreadsheet, their corresponding markdown files were left behind — the generation script only added/updated files, never removed them.

Changes

• content/glossary/_create_glossaries.py: Before writing markdown files for each language, remove the existing language directory entirely and recreate it fresh, so the output always mirrors the current spreadsheet state exactly.

# Remove existing directory to ensure deleted entries don't persist
if os.path.exists(language_dir):
    shutil.rmtree(language_dir)
os.makedirs(language_dir)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make deleted glossary pages go away in update</issue_title>
<issue_description>The data processing action can update glossary entries and then opens a PR. However, this only appears to add and change files - while it should also delete any files that are no longer generated. Assess the current behaviour and fix if needed - possibly by deleting the glossary folder before regeneration in the action.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Make deleted glossary pages go away in update #669

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