[docs] [C++20] [Modules] Ideas for transitioning to modules (#80687)

This patch tries to provide some ideas to transform an existing
libraries to modules. I feel this is helpful for users who is interested
in modules from my observation. While the syntax of modules look easy to
understand, the practice gets harder if the users want to make
compatible work with headers. Especially the `std` module can be
installed in clang18, I think such document may be helpful.

I tried to not be too wordy in this document and I don't want the users
to have impressions that they have to follow this or this is the best
practice. So I tried to use the term `idea` imply the users this is not
compulsory.

I add some regular reviewers for modules, but review opinions from users
are highly recommended.

I want to land this before the releasing of clang18 (in the early
March). We can and should improve this continuously.