Documentation: Supported boards and shields: Only show vendors with boards for specified architecture or vice versa

[Thalley]

Is your enhancement proposal related to a problem? Please describe.

On https://docs.zephyrproject.org/latest/boards/index.html if you select an architecture, then you can select vendors with 0 boards for that architecture.

For example, if I pick RISC-V as an architecture, then only 44/621 supported boards show up. If I'm interesting in finding a board from a specific vendor, then I already have to know that, as picking most vendors for RISC-V leads to an empty list.
Similarly, if I pick a vendor first, then most of the architectures will be empty

Describe the solution you'd like
When a architecture or a vendor is chosen, it should only be possible to select a architecture/vendor that results in non-0 boards. This could be done by either graying it out and make e.g. 96Boards unselectable if the architecture is RISC-V, or simply removing it from the dropdown list.

Describe alternatives you've considered
N/A

Additional context
An additional note is that it is not possible to "unselect" a choice. For example if I pick RISC-V and then a vendor, I'm not able to "unselect" the vendor again to get the list of all RISC-V boards. Ditto for architecture. The only way to get back is to reset all filters, which isn't too bad now, but when #79670 is merged with more filters, then it would be annoying having to re-select all options again.