[Docs] Add the allowed FilterableValues list to the Index docs page

[xDovos]

currently the Index docs page doesn't list which types can be indexed and which can't and because of #2493 it's really hard to figure out what the problem is if you don't know it. for example it is really important to know that floats aren't supported as it's just natural to try to make a multicolumn btree index on pos_x, pos_y(, pos_z) f32 data
other things that get tried often and don't work is Timestamps and timeDurations #2650