[tool, doc] Clarify autogenerated doc via "gen_cfg_md.py"

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The autogenerated file `/doc/interfaces.md` requires some changes to be clearer:

1. `rst_n` port is defined (nor `rst_shadowed_n` when required). Even if the link to the "comportability" doc is provided, I think it should be there to be homogeneous as `clk` is defined.
2. When `rst_shadowed_n` is declared, a link to explain this feature should also be provided.
3. The widths should refer to the actual bus widths as the RTL does, rather than to the _array size_ of the port.
4. The column _Act_ is supposed to give the direction of the port, but `rcv`, `req`, and `rsp` are not clear. We should probably move to a less fancy way: column name _Direction_ and possible values `input`, `output` or `inout`.

Start by reading gen_cfg_md.py and tracing how it generates /doc/interfaces.md. Check the generated interface documentation against the RTL port widths and reset declarations, then verify that reset links and a clear Direction column with input, output, or inout values appear in the output.

Written by the indexing model from the issue text.