Update Rust client SDK quickstart for 1.0 API

[gefjon]

A whole bunch of stuff has changed since this document was last updated. Notably, I've chosen to re-order a bunch of sections, since the previous structure of the document doesn't make much sense after the 0.12 API rework.

Assuming this gets approved, we should likely copy these structural changes into the TS and C# versions as well.

[rekhoff]

Read over document. It all makes sense and introduces the material well.

[jdetter]

I am following this quickstart as part of doing the release and I ran into a few simple issues. It currently doesn't build so please fix before merging:

Boppy@JASKIER MINGW64 ~/clockwork/quickstart-rust-sdk (master)
$ cargo run
   Compiling quickstart-rust-sdk v0.1.0 (C:\Users\Boppy\clockwork\quickstart-rust-sdk)
error[E0433]: failed to resolve: use of undeclared crate or module `credentials`
  --> src\main.rs:48:21
   |
48 | fn creds_store() -> credentials::File {
   |                     ^^^^^^^^^^^ use of undeclared crate or module `credentials`
   |
help: consider importing this module
   |
2  + use spacetimedb_sdk::credentials;
   |

error[E0433]: failed to resolve: use of undeclared crate or module `credentials`
  --> src\main.rs:49:5
   |
49 |     credentials::File::new("quickstart-chat")
   |     ^^^^^^^^^^^ use of undeclared crate or module `credentials`
   |
help: consider importing one of these structs
   |
2  + use std::fs::File;
   |
2  + use spacetimedb_sdk::credentials::File;
   |
help: if you import `File`, refer to it directly
   |
49 -     credentials::File::new("quickstart-chat")
49 +     File::new("quickstart-chat")
   |

warning: unused import: `spacetimedb_sdk::credentials::File`
 --> src\main.rs:3:5
  |
3 | use spacetimedb_sdk::credentials::File;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused variable: `ctx`
   --> src\main.rs:170:17
    |
170 | fn on_sub_error(ctx: &ErrorContext, err: Error) {
    |                 ^^^ help: if this is intentional, prefix it with an underscore: `_ctx`
    |
    = note: `#[warn(unused_variables)]` on by default

For more information about this error, try `rustc --explain E0433`.
warning: `quickstart-rust-sdk` (bin "quickstart-rust-sdk") generated 2 warnings
error: could not compile `quickstart-rust-sdk` (bin "quickstart-rust-sdk") due to 2 previous errors; 2 warnings emitted

[jdetter]

After my edit this works