let block example in docs is erroring -- remove outdated examples and test examples during doc build

The example in the docs https://juliadynamics.github.io/ResumableFunctions.jl/stable/manual/#Let-block is erroring although it appears working in the docs. ```julia julia> @resumable function arrays_of_tuples() for u in [[(1,2),(3,4)], [(5,6),(7,8)]] for i in 1:2 let i=i val = [a[i] for a in u] end @yield val end end end arrays_of_tuples (generic function with 1 method) julia> for array in arrays_of_tuples() println(array) end ERROR: UndefVarError: `val` not defined in `Main` Suggestion: check for spelling errors or missing imports. Stacktrace: [1] macro expansion @ ~/.julia/packages/ResumableFunctions/qmUpa/src/transforms.jl:250 [inlined] [2] (::var"##arrays_of_tuples_FSMI#367"{…})(_arg::Nothing) @ Main ~/.julia/packages/ResumableFunctions/qmUpa/src/macro.jl:110 [3] ##arrays_of_tuples_FSMI#367 @ ~/.julia/packages/ResumableFunctions/qmUpa/src/macro.jl:213 [inlined] [4] iterate (repeats 2 times) @ ~/.julia/packages/ResumableFunctions/qmUpa/src/types.jl:24 [inlined] [5] top-level scope @ ./REPL[48]:1 Some type information was truncated. Use `show(err)` to see complete types. ```