[doc] Render MUBI registers and reset values correctly

### Description

CSRs that are declared as a MUBI in Hjson are not declared as such in the rendered HTML table.
This makes it somewhat difficult to figure out what the reset values mean without looking at the Hjson source.
Example: various page attributes in the flash controller:
https://opentitan.org/book/hw/ip/flash_ctrl/data/flash_ctrl.html#mp_region_cfg

It would be good to address that in the future to improve the documentation experience.

CC @HU90m @jdonjdon

Start with the rendered flash controller page at the linked mp_region_cfg entry and compare its CSR table with the Hjson source. Identify the documentation-rendering entry point and any related tests; done means MUBI declarations and reset values are represented clearly in the generated HTML.

Written by the indexing model from the issue text.