guide: document #[pyo3(from_py_with)]

[davidhewitt]

Closes #1432

[birkenfeld]

There's a lot more here than the PR title suggests :)

[davidhewitt]

Heh sorry, I was a bit short of time when finishing up and posting this PR!

To describe what's going on, really the only new content is indeed a description for #[pyo3(from_py_with)].

The rest of the changes is just rearrangements to function.md which I thought made sense to support this.

Here's a full list for reviewers:

• Expanded the chapter opening with some links to the sub-content of the chapter.
• Moved the description of #[pyo3(text_signature = "...")] for classes and methods to class.md.
• Moved #[pyfn] docs to the bottom of function.md (and made description of #[pyfn] warn that I'd like to remove it).
• Moved the other #[pyo3(<option> = <value>)] bits into the bullets under "Function options".

[mejrs]

LGTM