RFC: Add description to load a local module and using it in example codes.

[AtsushiSakai]

I think current Modules doc does not explain how to load a local module and some example codes do not work correctly reported in #40384, because the some example codes do not use local module loading syntax (missing a dot before the module name).

So, I added a description to load a local module and using it in example codes to fix #29302 and to fix #40384.

[vtjnash]

Would you be willing to change these to doctests? This should show how to do it: https://juliadocs.github.io/Documenter.jl/stable/man/doctests/#Preserving-Definitions-Between-Blocks which mostly just means changing in the necessary places

```julia

to

```jldoctest nicestuff

[AtsushiSakai]

@vtjnash Thank you for your comment. I changed the related example codes to doctests.
I found some LinearAlgebra.jl doctests failed in my environment, I opened an issue as #41454.
But, the created doctests in this PR works correctly in my environment. PTAL.

[ViralBShah]

Let's rebase this to master, and hope that the whitespace check runs through.

Anything else we need here?

[AtsushiSakai]

I rebased.

[ViralBShah]

Whitespace check is failing

[AtsushiSakai]

Thank you. The remaining CI failures look unrelated.

[ViralBShah]

Is this good to merge?

[AtsushiSakai]

For me, yes.