[ EPIC ] Adding Transparency to the Transpiler

[kaelynj]

URL, if applicable

No response

Describe the new content you are requesting.

From the discussion in #878, I've identified a few topics for new pages/edits to other pages that I will work on adding in to the transpile section. I'll keep track of the list of changes using this issue and will expand the list as needed. Feel free to add suggestions or comments.

- [ ] https://github.com/Qiskit/documentation/issues/1071
- [ ] https://github.com/Qiskit/documentation/issues/1075
- [ ] https://github.com/Qiskit/documentation/issues/1078
- [ ] Edit the `qiskit.synthesis` [overview docstring](https://docs.quantum.ibm.com/api/qiskit/synthesis) to make more clear what the module is designed for (from what I understand it's the "engine" powering some/all of the transformation passes, among other things).
- [ ] Expand introduction to the `qiskit.transpiler.passes` [overview section](https://docs.quantum.ibm.com/api/qiskit/transpiler_passes) in the API docs.
- [ ] https://github.com/Qiskit/documentation/issues/1077
- [ ] https://github.com/Qiskit/documentation/issues/891

If this new content request is accepted, do you want to write the content?

I will write a draft of the proposed content