Remove #[doc(hidden)] from stable crates (#3226)

This PR removes `#[doc(hidden)]` from types/functions in stable crates
(as defined in
[CrateSet.kt](https://github.com/smithy-lang/smithy-rs/blob/ad520b080aa0a3e8239ab2857dafb519c3458088/buildSrc/src/main/kotlin/CrateSet.kt#L19-L33)

).
They are now `pub`, however, for those that are not intended to be used
directly, we preserve the original docs to be explicit about it.

After this PR, stable crates contain neither
<details>
<summary>#[doc(hidden)]</summary>

```
➜  smithy-rs git:(ysaito/remove-doc-hidden) rg -l '#\[doc\(hidden\)\]'
aws/rust-runtime/aws-http/src/user_agent.rs
CHANGELOG.next.toml
CHANGELOG.md
rust-runtime/aws-smithy-http/src/lib.rs
rust-runtime/aws-smithy-http/src/event_stream/sender.rs
rust-runtime/aws-smithy-http/src/event_stream/receiver.rs
codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/error/ErrorImplGenerator.kt (this is only for CodegenTarget.SERVER)
codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/StructureGeneratorTest.kt
design/src/server/instrumentation.md
design/src/rfcs/rfc0039_forward_compatible_errors.md
design/src/rfcs/rfc0032_better_constraint_violations.md
design/src/rfcs/rfc0026_client_crate_organization.md
rust-runtime/aws-smithy-http-server/src/macros.rs
rust-runtime/aws-smithy-http-server/src/routing/mod.rs
rust-runtime/aws-smithy-http-server/src/shape_id.rs
rust-runtime/aws-smithy-http-server/src/body.rs
rust-runtime/aws-smithy-http-server/src/lib.rs
rust-runtime/aws-smithy-http-server/src/plugin/mod.rs
codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/MapConstraintViolationGenerator.kt
codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/CollectionConstraintViolationGenerator.kt
```
</details>

nor

<details>
<summary>DocHidden</summary>

```
➜  smithy-rs git:(ysaito/remove-doc-hidden) rg -l 'DocHidden'
codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt
codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/ServerBuilderConstraintViolations.kt
```
</details>

<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: Russell Cohen <rcoh@amazon.com>