Document how to use CF data in factor evaluation

[dehann]

taking over from #783

xref #467

[dehann]

function (cf::CalcFactor<:LinearRelative})(z, x1, x2)
  # cf.metadata
  #  ...
end

[dehann]

Minimum docs here: https://juliarobotics.org/Caesar.jl/latest/examples/basic_definingfactors/#Metadata

[Affie]

FMD is deprecated. So we should document replacements is calcfactor

[dehann]

I've added quite a bit of documentation to the types CCW and CalcFactor which should already help a lot. Still worth adding a bit to Caesar docs on own factors just for quicker learning curve.