Spatial hash docs update

[fluidnumericsJoe]

Description

• Update unstructured grid search docs to reflect variable bitwidth in construction
• Documentation on use of the gnomonic projection, rather than planar normal projection ( PIC check algorithm adjustment )
• Physical element - hash grid cell bounds for spherical mesh types

[erikvansebille]

WHta's the status on this PR, @fluidnumericsJoe? Should we get this merged soon? It won't be in the official release, but would be good to get it on the website asap.

But before we merge: is it useful also adding some info on #2883 in this documentation?

[fluidnumericsJoe]

Thanks for the update here @wyatt-fluidnumerics - I'm going to run another pass through to make sure we're all nice and consistent with everything implemented between spatialhash and index search. On a first glance, I'm wanting to fill in a bit more detail on curvilinear coordinate mapping (bilinear interpolation). Seems we could also use local links to other sections in a few spots to make navigation easier