Need a section about "shared source" and update the "Source" entry in glossary

[hengm3467]

Related code PR

#2449 (comment)

Which part(s) of the docs might be affected or should be updated? And how?

I see the concept of sources have evolved in RW, and now we need to talk about shared sources frequently. I feel we need to explain the definition and purpose of shared sources, so that we can point to it from elsewhere.

Reference

No response