Docs: add minimal examples of a full optimization & CachingOptimizer

[matbesancon]

There should be some examples in the documentation on the use of MOIU.CachingOptimizer, without that it is hard to get an entry point into MOI (make something work without JuMP).

Cbc for instance is not usable without MOIU.CachingOptimizer or JuMP.

[matbesancon]

To complement on this issue, errors appearing seem inconsistent with the docstrings and error messages:

julia> o = SCS.Optimizer()
julia> z = MOI.add_variables(o, 3)
ERROR: MathOptInterface.AddVariableNotAllowed: Adding variables cannot be performed. You may want to use a `CachingOptimizer` in `AUTOMATIC` mode or you may need to call `reset_optimizer` before doing this operation if the `CachingOptimizer` is in `MANUAL` mode.
julia> model = MOI.Utilities.CachingOptimizer(o, MOI.Utilities.AUTOMATIC)
MOIU.CachingOptimizer{MOI.AbstractOptimizer,SCS.Optimizer}
in state NO_OPTIMIZER
in mode AUTOMATIC
with model cache SCS.Optimizer
with optimizer nothing

The last command is the first working method that comes to mind following the thrown error but is likely wrong: a user seeing an error because the optimizer does not support incremental changes will likely want to build a caching optimizer with whatever default works as a cache. There is no indication of what object is expected as model cache (if the user does not have another Optimizer type at hand that supports it.

[matbesancon]

UniversalFallback should IMO be mentioned or even used as cache in a default method

[odow]

Closed by #1272