Update docs for raw FFI use

[ngoldbaum]

I noticed some issues in these docs while working on #4588. Opening this PR so that one doesn't get bogged down.

• Updates the listing of build config variables in the top-level API doc page
• Moves raw FFI examples into a new examples folder in the pyo3-ffi crate.
• Updates the top-level pyo3-ffi docs to pull the content of the string-sum example from the examples folder. The content that is there now is substantially out of sync from the content in the examples folder and that seemed wrong to me.
• Added a note in the narrative docs about the sequential example and mention multi-phase initialization in the top-level pyo3-ffi docs.

[davidhewitt]

Thanks, this is a good improvement. I spotted few small additional refinements:

[ngoldbaum]

I think check-guide is failing because there isn't a tagged release yet where the new examples exist:

/Users/goldbaum/Documents/pyo3/target/doc/pyo3_ffi/index.html]:
✗ [404] https://github.com/PyO3/pyo3/tree/v0.23.0-dev/pyo3-ffi/examples/sequential | Failed: Network error: Not Found

Not sure how to work around or ignore that.

[ngoldbaum]

Not sure how to work around or ignore that.

Removed the links, we can always add better cross-referencing later.

[davidhewitt]

Fair enough, sorry for the slow reply by me. The check-guide job in the noxfile has some mapping for various locally-built files; probably we could have added to that.

This looks good to me, thanks 👍