WIP: Create API reference

[TedDriggs]

This reference lists all supported attributes alphabetically.
It should in future link to examples and higher-level documentation.

Part of #232

[TedDriggs]

@andy128k can you take a look at this? I'm going to keep working away at getting all the options captured, and would appreciate a second set of eyes looking for errors and omissions.

[andy128k]

I think it would be hard to maintain such document unless it is managed/covered by cargo doc.

[TedDriggs]

I wish there was a way to use cargo doc over our options structs to auto-generate this, but the options structs aren't public; derive_builder_core deliberately only exposes builder_for_struct(DeriveInput) -> TokenStream so that adding new options doesn't force us to rev the crate version.

Even serde doesn't seem to have a good way of generating this documentation; it's kept separately (example)

The current approach with a flat list of features in README.md isn't working - see #224 and #225 as recent examples where that's failed.

I'm hoping that the rate of API changes for derive_builder will be slow going forward; the crate seems pretty mature, and I'm increasingly conservative in what changes we should accept. I therefore think it'll be possible to manually enforce that any macro API change PR must have an update to this file.

[indietyp]

There are also things like: https://docs.rs/binread/latest/binread/attribute/index.html, which create an additional documentation-only module.