feat: add specialized footnotes and endnotes toolset

[KeithCu]

This PR introduces a specialized toolset for Footnotes and Endnotes within LibreOffice Writer. It follows the "skinny but full-featured" API design preference, implementing granular tools footnotes_insert, footnotes_list, footnotes_edit, and footnotes_delete.

It includes:

• Research and API documentation in docs/footnotes-api-reference.md.
• Base classes in plugin/modules/writer/base.py.
• Tool implementations in plugin/modules/writer/footnotes.py
• Integration in plugin/modules/writer/__init__.py.
• Test suite in plugin/tests/test_writer_footnotes.py.

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PR created automatically by Jules for task 4172254140045783599 started by @KeithCu

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