Develop wiki/vignette/chapter of user guide on mesh guidance

[seananderson]

• gather useful resources from INLA
• provide some practical guidance
• 'quick' make_mesh() versions vs. all the options in INLA
• guidance on when to bother with barriers (and note anisotropy often more important)
• how the mesh can affect convergence
• finer meshes aren't always better for prediction
• tweaks to usual INLA advice given TMB is faster for moderate meshes but doesn't scale as well to very high resolutions
• what the INLA arguments mean for humans
• plotting
• a reference we can point to rather than giving the same advice over and over