[otbn] Duplicated error code list

The list of OTBN error codes is duplicated, appearing in these 3 places:
- `otbn_pkg.sv`
- The HTML documentation
- The ISS (`alert.py`)

We should probably list them in the hjson and generate the other files. Easy enough for the documentation and ISS. For the SV code, maybe we could get reggen to generate the list, and then move the list to `otbn_reg_pkg.sv`.

Compare the duplicated error-code lists in otbn_pkg.sv, the HTML documentation, and the ISS implementation in alert.py, then inspect the relevant HJSON and reggen paths. Define one source of truth and verify that the documentation, ISS, and SystemVerilog outputs remain consistent; the issue is done when the duplicated lists are generated or removed as appropriate.

Written by the indexing model from the issue text.