Accept .dotx and .dotm Word templates #83

Document() accepts .docx and, since f49bdd1, macro-enabled .docm. Word template files — .dotx and .dotm — are still rejected, because their main-part content types are not in _WORD_MAIN_CONTENT_TYPES in docx/api.py.

Why it matters

Templates are the natural starting point for document generation: an organisation keeps its house styles, headers and numbering in a .dotx, and a pipeline should be able to open it, fill it in, and save a .docx. Today that requires renaming the file and hoping, or opening the template in Word once to convert it.

The markup inside a template is identical to a document — only the content type differs — so this is a small change, exactly parallel to the .docm support already merged.

Upstream python-openxml#363 (16 comments, open since 2017) and python-openxml#1532, with unmerged PR python-openxml#1537.

Scope

• Add the template content types to _WORD_MAIN_CONTENT_TYPES in docx/api.py:
  application/vnd.openxmlformats-officedocument.wordprocessingml.template.main+xml and the macro-enabled template equivalent
• Decide save behaviour, which is the only real design question: saving a .dotx unchanged keeps the template content type, so the output is still a template. Generating a document from a template means rewriting the main part content type on save. Both are legitimate; the API should let the caller choose rather than picking silently
• Check whether the [Content_Types].xml override and the docProps need adjusting when converting template → document
• Follow the .docm commit as the model, including its test approach
• Unit tests for open, save-as-template and save-as-document, plus fixture files
• Update the Document() docstring, which currently names only .docx and .docm

Notes

Related: #23 (Word default template) concerns the bundled default.docx, which is a different problem — this issue is about opening user-supplied templates.