guide: fixes and some minor additions (#802)

* guide: fixes and some minor additions

I went through the guide and fixed some grammar/consistency issues,
and added some wording where I thought it appropriate.

I also reordered the chapters to what I feel is a more natural
flow of concepts.

* Apply suggestions from code review and fix tests

Co-Authored-By: Yuji Kanagawa <yuji.kngw.80s.revive@gmail.com>
Co-Authored-By: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

Co-authored-by: Yuji Kanagawa <yuji.kngw.80s.revive@gmail.com>
Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

guide/src/SUMMARY.md

@@ -1,16 +1,16 @@
 # Summary
 
-- [Get Started](get_started.md)
+- [Getting Started](get_started.md)
+- [Python Modules](module.md)
+- [Python Functions](function.md)
+- [Python Classes](class.md)
 - [Type Conversions](conversions.md)
-- [Python Exception](exception.md)
-- [Python Module](module.md)
-- [Python Function](function.md)
-- [Python Class](class.md)
-- [Call Python from Rust](python_from_rust.md)
+- [Python Exceptions](exception.md)
+- [Calling Python from Rust](python_from_rust.md)
 - [Parallelism](parallelism.md)
 - [Debugging](debugging.md)
 - [Advanced Topics](advanced.md)
 - [Building and Distribution](building_and_distribution.md)
 - [PyPy support](pypy.md)
 - [Appendix A: PyO3 and rust-cpython](rust_cpython.md)
-- [Appendix B: Migration Guide](migration.md)
+- [Appendix B: Migration Guide](migration.md)

guide/src/advanced.md

@@ -1,6 +1,6 @@
 # Advanced topics
 
-## ffi
+## FFI
 
 PyO3 exposes much of Python's C API through the `ffi` module.