Add a Sphinx extension (docs/pysim_fs_sphinx.py) that hooks into the builder-inited event and generates docs/filesystem.rst before Sphinx reads any source files.
The generated page contains a hierarchical listing of all implemented EFs and DFs, organised by application/specification (UICC/TS 102 221, ADF.USIM/TS 31.102, ADF.ISIM/TS 31.103, SIM/TS 51.011). For each file, the class docstring and any _test_de_encode / _test_decode vectors are included as an encoding/decoding example table.
docs/filesystem.rst is fully generated at build time and is therefore added to .gitignore.
