Subxt Guide (#890)

* WIP Starting to write book; extrinsics first pass done

* cargo fmt

* Ongoing work; events, constants, wip blocks

* at_latest() and wip blocks

* remove need to import parity-scale-codec crate with Subxt for macro to work

* More docs; expanding on setup guide and finish pass of main sections

* Tidy and remove example section for now

* format book lines to 100chars

* Fix example code

* cargo fmt

* cargo fmt

* fix example

* Fix typos

* fix broken doc links, pub mods

* Update Subxt macro docs

* can't link to Subxt here

* move macro docs to Subxt to make linking better and fix example code

* note on macro about docs

* cargo fmt

* document the no_default_derives macro feature

* Address feedback and remove redundant text

* address review comments; minor tweaks

* WIP add Runtime calls to book

* Improve Runtime API docs

* expose thing we forgot to expose and doc link fixes

subxt/src/dynamic.rs

@@ -11,7 +11,7 @@ use crate::{
 };
 use scale_decode::DecodeAsType;
 
-pub use scale_value::Value;
+pub use scale_value::{At, Value};
 
 /// A [`scale_value::Value`] type endowed with contextual information
 /// regarding what type was used to decode each part of it. This implements