Update the Split Release Process RFC (#992)

RFC: Split Release Process

==========================

> Status: RFC

> Status: Implemented in [smithy-rs#986](https://github.com/awslabs/smithy-rs/pull/986) and [aws-sdk-rust#351](https://github.com/awslabs/aws-sdk-rust/pull/351)

At the time of writing, the `aws-sdk-rust` repository is used exclusively

for the entire release process of both the Rust runtime crates from `smithy-rs` as

-----------------

- In `smithy-rs`:

  - [ ] Move publisher tool from `aws-sdk-rust` into `smithy-rs`

  - [ ] Modify `aws:sdk:assemble` target to run the publisher `fix-manifests` subcommand

  - [ ] Add `rust-runtime:assemble` target that generates publish-ready Smithy runtime crates

  - [ ] Add CI step to create Smithy runtime bundle artifact

  - [ ] Add `GITHUB_ACTIONS=true` env var check for setting the `publish` flag in generated AND runtime manifests

  - [ ] Revise publisher tool to publish from an arbitrary directory

  - [x] Move publisher tool from `aws-sdk-rust` into `smithy-rs`

  - [x] Modify `aws:sdk:assemble` target to run the publisher `fix-manifests` subcommand

  - [x] Add `rust-runtime:assemble` target that generates publish-ready Smithy runtime crates

  - [x] Add CI step to create Smithy runtime bundle artifact

  - [x] Add `GITHUB_ACTIONS=true` env var check for setting the `publish` flag in generated AND runtime manifests

  - [x] Revise publisher tool to publish from an arbitrary directory

- In `aws-sdk-rust`:

  - [ ] Implement CI for the `aws-sdk-rust/next` branch

  - [ ] Remove the publisher tool

- [ ] Update release process documentation

  - [x] Implement CI for the `aws-sdk-rust/next` branch

  - [x] Remove the publisher tool

- [x] Update release process documentation